Oxide semiconductor film and semiconductor device

ABSTRACT

To provide a novel oxide semiconductor film. The oxide semiconductor film includes In, M, and Zn. The M is Al, Ga, Y, or Sn. In the case where the proportion of In in the oxide semiconductor film is 4, the proportion of M is greater than or equal to 1.5 and less than or equal 2.5 and the proportion of Zn is greater than or equal to 2 and less than or equal to 4.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, oneembodiment of the present invention relates to an oxide semiconductorfilm or a manufacturing method of the oxide semiconductor film. Inparticular, one embodiment of the present invention relates to asemiconductor device, a display device, a liquid crystal display device,a light-emitting device, a power storage device, a memory device, amethod for driving them, or a method for manufacturing them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

BACKGROUND ART

Non-Patent Document 1 discloses that a homologous series represented byIn_(1−x)Ga_(1+x)O₃(ZnO)_(m) (−1≤x≤, and m is a natural number) exists.Furthermore, Non-Patent Document 1 discloses a solid solution range of ahomologous series. For example, in the case of a solid solution range ofa homologous series when m is 1, x is within a range from −0.33 to 0.08,and in the case of a solid solution range of a homologous series when mis 2, x is within a range from −0.68 to 0.32.

Furthermore, a technique in which a transistor is fabricated using anIn—Ga—Zn-based oxide semiconductor is disclosed (for example, see PatentDocument 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2007-96055

-   Non-Patent Document-   [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., Vol. 93, 1991, pp. 298-315.

DISCLOSURE OF INVENTION

Non-Patent Document 1 discloses an example of In_(x)Zn_(y)Ga_(z)O_(w),and when x, y, and z are set so that a composition in the neighborhoodof ZnGa₂O₄ is obtained, i.e., x, y, and z are close to 0, 1, and 2,respectively, a spinel crystal structure is likely to be formed ormixed. A compound represented by AB₂O₄ (A and B are metal) is known as acompound having a spinel crystal structure.

However, when a spinel crystal structure is formed or mixed in anIn—Ga—Zn-based oxide semiconductor film, electrical characteristics orreliability of a semiconductor device (e.g., a transistor) including theIn—Ga—Zn-based oxide semiconductor film is adversely affected by thespinel crystal structure in some cases.

In view of the above problem, an object of one embodiment of the presentinvention is to provide a novel oxide semiconductor film. Another objectis to give favorable electrical characteristics to a semiconductordevice. Another object is to provide a highly reliable semiconductordevice. Another object is to provide a semiconductor device with a novelstructure. Another object is to provide a display device having a novelstructure.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is an oxide semiconductor filmincluding In, M (M is Al, Ga, Y, or Sn), and Zn, having a composition inthe neighborhood of a solid solution range of anIn_(1+x)M_(1−x)O₃(ZnO)_(y) structure (x satisfies 0<x<0.5, and y isapproximately 1). In the above embodiment, x is preferably approximately0.33.

In the above embodiments, it is preferable that the oxide semiconductorfilm have an atomic ratio in a neighborhood of In:M:Zn=4:2:3 and in thecase where the proportion of In is 4, the proportion of M be greaterthan or equal to 1.5 and less than or equal to 2.5 and the proportion ofZn be greater than or equal to 2 and less than or equal to 4.

In the above embodiments, it is preferable that the oxide semiconductorfilm be formed with a sputtering apparatus, the sputtering apparatusinclude a polycrystalline metal oxide target, the polycrystalline metaloxide target have a composition in the neighborhood of In:M:Zn=4:2:4.1,and in the case where the proportion of In of the polycrystalline metaloxide target is 4, the proportion of M be greater than or equal to 1.5and less than or equal to 2.5 and the proportion of Zn be greater thanor equal to 3.1 and less than or equal to 5.1.

In the above embodiments, it is preferable that the oxide semiconductorfilm include a crystal part and that the crystal part have c-axisalignment.

In the above embodiments, the oxide semiconductor film preferablyincludes a region with a hydrogen concentration of lower than 1×10²⁰atoms/cm³. In the above embodiments, the oxide semiconductor filmpreferably includes a region where In, M, Zn, and O atoms in totalaccount for 99.97 atomic % or more. In the above embodiments, the oxidesemiconductor film preferably includes a region where Fe, Ni, and Siatoms in total account for lower than 0.03 atomic %.

Another embodiment of the present invention is an oxide semiconductorfilm including In, M (M is Al, Ga, Y, or Sn), and Zn. The oxidesemiconductor film includes a first oxide semiconductor film and asecond oxide semiconductor film over the first oxide semiconductor film.The first oxide semiconductor film has a composition in the neighborhoodof a solid solution range of the In_(1+x)M_(1−x)O₃(ZnO)_(y) structure (xsatisfies 0<x<0.5, and y is approximately 1). The second oxidesemiconductor film includes a region having a composition in theneighborhood of a solid solution range of the In_(1+x)M_(1−v)O₃(ZnO)_(w)structure (v satisfies −0.2≤v<0.2, and w is approximately 1) and havinga smaller proportion of indium atoms than the first oxide semiconductorfilm.

In the above embodiment, x is preferably approximately 0.33 and v ispreferably approximately 0.

In the above embodiments, it is preferable that the first oxidesemiconductor film have an atomic ratio in the neighborhood ofIn:M:Zn=4:2:3, the second oxide semiconductor film have an atomic ratioin the neighborhood of In:M:Zn=1:1:1, and in the case where theproportion of In of the first oxide semiconductor film is 4, theproportion of M be greater than or equal to 1.5 and less than or equalto 2.5 and the proportion of Zn be greater than or equal to 2 and lessthan or equal to 4.

In the above embodiments, it is preferable that the first oxidesemiconductor film and the second oxide semiconductor film be formedwith a sputtering apparatus, the sputtering apparatus include a firstpolycrystalline metal oxide target and a second polycrystalline metaloxide target, the first polycrystalline metal oxide target have acomposition in the neighborhood of In:M:Zn=4:2:4.1, the secondpolycrystalline metal oxide target have a composition in theneighborhood of In:M:Zn=1:1:1.2, and in the case of the proportion ofthe In of the first polycrystalline metal oxide target is 4, theproportion of M be greater than or equal to 1.5 and less than or equalto 2.5 and the proportion of Zn be greater than or equal to 3.1 and lessthan or equal to 5.1.

In the above embodiments, the first oxide semiconductor film preferablyincludes a crystal part, and the crystal part preferably has c-axisalignment.

In the above embodiments, the first oxide semiconductor film preferablyincludes a region with a hydrogen concentration of lower than 1×10²⁰atoms/cm³. In the above embodiments, the first oxide semiconductor filmpreferably includes a region where In, M, Zn, and O atoms in totalaccount for 99.97 atomic % or more. In the above embodiments, the firstoxide semiconductor film preferably includes a region where Fe, Ni, andSi atoms in total account for lower than 0.03 atomic %.

Another embodiment of the present invention is a semiconductor deviceincluding any one of the above-described oxide semiconductor films, agate insulating film in contact with the oxide semiconductor film, agate electrode in contact with the gate insulating film, a sourceelectrode electrically connected to the oxide semiconductor film, and adrain electrode electrically connected to the oxide semiconductor film.

Another embodiment of the present invention is a display deviceincluding any one of the above oxide semiconductor films and a displayelement. Another embodiment of the present invention is a display moduleincluding the display device and a touch sensor. Another embodiment ofthe present invention is an electronic device including any one of theoxide semiconductor films, the semiconductor device, the display device,or the display module; and an operation key or a battery.

According to one embodiment of the present invention, a novel oxidesemiconductor film can be provided. According to one embodiment of thepresent invention, a semiconductor device can be provided with favorableelectrical characteristics. A highly reliable semiconductor device canbe provided. A semiconductor device with a novel structure can beprovided. A display device with a novel structure can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an atomic ratio of an oxide semiconductor film.

FIGS. 2A and 2B are phase equilibrium diagrams.

FIG. 3 shows the relation between the atomic ratio of the target and apercentage of remaining Zn.

FIGS. 4A to 4D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS.

FIGS. 5A to 5D are Cs-corrected high-resolution TEM images of a plane ofa CAAC-OS.

FIGS. 6A to 6C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 7A and 7B show electron diffraction patterns of a CAAC-OS.

FIG. 8 shows a change in the crystal part of an In—Ga—Zn oxide inducedby electron irradiation.

FIGS. 9A to 9C illustrate a deposition method of a CAAC-OS.

FIG. 10 illustrates a crystal of InMZnO₄.

FIGS. 11A to 11F illustrate a deposition method of a CAAC-OS.

FIGS. 12A to 12G illustrate positions to which particles can be attachedin a pellet.

FIGS. 13A to 13G illustrate positions to which particles can be attachedin a pellet.

FIGS. 14A to 14C are a top view and cross-sectional views illustratingan example of a semiconductor device.

FIGS. 15A to 15C are a top view and cross-sectional views illustratingan example of a semiconductor device.

FIGS. 16A to 16C are a top view and cross-sectional views illustratingan example of a semiconductor device.

FIGS. 17A to 17C are a top view and cross-sectional views illustratingan example of a semiconductor device.

FIGS. 18A to 18D are cross-sectional views illustrating examples of asemiconductor device.

FIGS. 19A and 19B each illustrate a band structure.

FIGS. 20A to 20D are cross-sectional views illustrating examples of asemiconductor device.

FIGS. 21A to 21F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 22A to 22F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 23A to 23F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 24A and 24B are model diagrams illustrating oxygen moving in anoxide semiconductor film.

FIGS. 25A to 25F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 26A to 26F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 27A and 27B illustrate a sputtering apparatus.

FIGS. 28A and 28B illustrate a sputtering apparatus.

FIG. 29A illustrates a sputtering apparatus and FIGS. 29B and 29C showpotential distributions of the vicinity of a sputtering target.

FIG. 30 illustrates a sputtering apparatus.

FIG. 31 illustrates a sputtering apparatus.

FIGS. 32A and 32B illustrate a sputtering apparatus.

FIG. 33 is a top view which illustrates an example of a depositionapparatus.

FIGS. 34A to 34C are cross-sectional views which illustrate an exampleof a deposition apparatus.

FIGS. 35A to 35C are a block diagram and circuit diagrams illustrating adisplay device.

FIGS. 36A and 36B are perspective views illustrating an example of atouch panel.

FIGS. 37A and 37B are cross-sectional views illustrating examples of adisplay device.

FIG. 38 is a cross-sectional view illustrating an example of a touchsensor.

FIGS. 39A and 39B are each a cross-sectional view of an example of atouch panel.

FIGS. 40A and 40B are a block diagram and a timing chart of a touchsensor.

FIG. 41 is a circuit diagram of a touch sensor.

FIG. 42 illustrates a display module.

FIGS. 43A to 43G illustrate electronic devices.

FIGS. 44A and 44B are perspective views of a display device.

FIG. 45 illustrates a structure of a deposition apparatus.

FIG. 46 is a graph showing XRD results of Samples of Example.

FIGS. 47A and 47B are cross-sectional TEM images of Samples of Example.

FIG. 48 is a graph showing measurement coordinate points of Samples ofExample.

FIG. 49 is a graph showing XRD results of Samples of Example.

FIG. 50 is a graph showing XRD results of Samples of Example.

FIG. 51 is a graph showing XRD results of Samples of Example.

FIG. 52 is a graph showing XRD results of Samples of Example.

FIG. 53 is a graph showing XRD results of Samples of Example.

FIG. 54 is a graph showing SIMS results of Samples of Example.

FIGS. 55A and 55B show Id-Vg characteristics of transistors of Example.

FIG. 56 shows results of gate BT stress tests performed on transistorsof Example.

FIGS. 57A to 57C show Id-Vg characteristics of transistors of Example.

FIGS. 58A and 58B are cross-sectional TEM images of a transistor ofExample.

FIG. 59 shows Id-Vg characteristics of transistors of Example.

FIG. 60 shows results of gate BT stress tests performed on a transistorof Example.

FIGS. 61A and 61B show Id-Vd characteristics of transistors of Example.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments and the examples.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over”, “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Furthermore, the positional relation between components ischanged as appropriate in accordance with a direction in which eachcomponent is described. Thus, there is no limitation on terms used inthis specification, and description can be made appropriately dependingon the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain, the channel region, and the source. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Note that in this specification and the like, a “silicon oxynitridefilm” refers to a film that includes oxygen at a higher proportion thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes nitrogen at a higher proportion than oxygen.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different diagrams are commonly denoted by the samereference numeral in some cases.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, the term “substantially parallel” indicatesthat the angle formed between two straight lines is greater than orequal to −30° and less than or equal to 30°. In addition, the term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly also includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°. A term “substantiallyperpendicular” indicates that the angle formed between two straightlines is greater than or equal to 60° and less than or equal to 120°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other depending on the case. For example, theterm “conductive layer” can be changed into the term “conductive film”in some cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Furthermore, a “semiconductor” and an “insulator” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called an “insulator” insome cases. Similarly, an “insulator” in this specification can becalled a “semiconductor” in some cases.

Embodiment 1

In this embodiment, an oxide semiconductor film which is one embodimentof the present invention is described.

The oxide semiconductor film of one embodiment of the present inventionincludes indium (In), M (M is Al, Ga, Y, or Sn), and zinc (Zn).Specifically, M is preferably gallium (Ga). In the followingdescription, Ga is used as M.

An oxide semiconductor film containing In has high carrier mobility(electron mobility), for example. An oxide semiconductor film has highenergy gap (Eg) by containing Ga, for example. Note that Ga is anelement having high bonding energy with oxygen, which is higher than thebonding energy of In with oxygen. When containing Zn, the oxidesemiconductor film is easily crystallized.

Here, the oxide semiconductor film of one embodiment of the presentinvention is described with reference to FIG. 1 , FIGS. 2A and 2B, FIG.3 , FIGS. 4A to 4D, FIGS. 5A to 5D, FIGS. 6A to 6C, FIGS. 7A and 7B,FIG. 8 , FIGS. 9A to 9C, FIG. 10 , FIGS. 11A to 11F, FIGS. 12A to 12G,and FIGS. 13A to 13G.

<1-1. Phase Equilibrium Diagram>

FIG. 1 is an example of a phase equilibrium diagram showing the atomicratios of the oxide semiconductor film of one embodiment of the presentinvention.

FIG. 1 illustrates a preferable range of the atomic ratio of In, M, andZn included in the oxide semiconductor film of one embodiment of thepresent invention. Note that the proportion of oxygen atoms is not shownin FIG. 1 .

The oxide semiconductor film of one embodiment of the present inventionis an oxide semiconductor film including In, M, and Zn. The compositionof the oxide semiconductor film is in the neighborhood of a solidsolution range of an In_(1+x)M_(1−x)O₃(ZnO)_(y) structure (x satisfies0<x<0.5, and y is approximately 1). That is, the oxide semiconductorfilm is an oxide semiconductor film having a composition in theneighborhood of Area 11 in FIG. 1 .

Examples of a method for forming the oxide semiconductor film of oneembodiment of the present invention include a sputtering method, apulsed laser deposition (PLD) method, a plasma-enhanced chemical vapordeposition (PECVD) method, a thermal chemical vapor deposition (CVD)method, an atomic layer deposition (ALD) method, and a vacuumevaporation method. As an example of a thermal CVD method, a metalorganic chemical vapor deposition (MOCVD) method can be given. Inparticular, the oxide semiconductor film of one embodiment of thepresent invention is preferably formed with a sputtering apparatusbecause a uniform film can be formed even over a large-sized glasssubstrate or the like.

Here, the atomic ratio of the elements in the phase equilibrium diagramis described with reference to FIGS. 2A and 2B. FIGS. 2A and 2B eachshow a regular triangle with vertices X, Y, and Z, and a coordinatepoint R (4:2:1) as an example. The vertices denote elements X, Y, and Z.As the distance to each vertex is reduced, the proportion of atoms ofthe corresponding element is increased, whereas as the distance isincreased, the proportion of the atoms is reduced. In addition, as shownin FIG. 2A, the proportion of atoms of each element is represented bythe length of a perpendicular line from the coordinate point to thesubtense of the corresponding vertex of the triangle. For example, theproportion of atoms of the element X is represented by the length of aperpendicular line 21 from the coordinate point to the subtense of thevertex X, i.e., to a side YZ. Therefore, the coordinate point R in FIGS.2A and 2B represents that the atomic ratio of the element X to theelement Y and the element Z is the ratio of the length of theperpendicular line 21 to the length of a perpendicular line 22 and thelength of a perpendicular line 23, that is, x:y:z=4:2:1. Furthermore, anintersection point of the side YZ with a line passing through the vertexX and the coordinate point R is denoted by γ. Assuming that the ratio ofthe length of a line Yγ to the length of a line γZ is Yγ:γZ, Yγ:γZcorresponds to (the number of atoms of the element Z):(the number ofatoms of the element Y).

As shown in FIG. 2B, three lines each of which passes through thecoordinate point R and is parallel to any of the three sides of thetriangle are drawn. By using intersection points of the three lines andthe three sides, x, y, and z can be represented as shown in FIG. 2B.

Note that as disclosed in Non-Patent Document 1, it is known that thereis a homologous series represented by InMO₃(ZnO)_(m) (m is a naturalnumber) as one of oxides containing In, the element M, and Zn. It isknown that regions denoted by thick lines in FIG. 1 allow a single-phasesolid solution range when powders of In₂O₃, M₂O₃, and ZnO are mixed andsintered at 1350° C.

Specifically, in the phase equilibrium diagram in FIG. 1 , broken linesindicate a solid solution range of the In:M:Zn=(1+α):(1−α):1 structure(a satisfies −1≤α≤1), a solid solution range of theIn:M:Zn=(1+α):(1−α):2 structure, a solid solution range of theIn:M:Zn=(1+α):(1−α):3 structure, a solid solution range of theIn:M:Zn=(1+α):(1−α):4 structure, and a solid solution range of theIn:M:Zn=(1+α):(1−α):5 structure. On dashed-dotted lines, the atomicratios of In to Mare 1:1, 1:2, 1:3, 1:7, 2:1, 3:1, and 5:1.

A coordinate point denoted by a square in FIG. 1 corresponds to ZnM₂O₄that is a known composition with which a spinel crystal structure islikely to be mixed. A coordinate point A denoted by a black circle inFIG. 1 indicates the composition In:M:Zn=1.33:0.67:1 (aroundIn:M:Zn=4:2:3). A coordinate point B denoted by a triangle in FIG. 1indicates the composition In:M:Zn=1:1:1. A two-dotted line in FIG. 1corresponds to a straight line connecting a coordinate point indicatingIn and the coordinate point indicating ZnM₂O₄. In Area 12 in FIG. 1 , aspinel crystal structure is likely to be mixed.

In Non-Patent Document 1, the solid solution range of theIn:M:Zn=(1+α):(1−α):1 structure, the solid solution range of theIn:M:Zn=(1+α):(1−α):2 structure, the solid solution range of theIn:M:Zn=(1+α):(1−α):3 structure, the solid solution range of theIn:M:Zn=(1+α):(1−α):4 structure, and the solid solution range of theIn:M:Zn=(1+α):(1−α):5 structure in FIG. 1 are those in the equilibriumstates obtained under the case where powders of In₂O₃, M₂O₃, and ZnO aremixed and sintered at 1350° C.

Meanwhile, an oxide semiconductor film formed with a sputteringapparatus or the like may be in a quasi-equilibrium state in the casewhere high-temperature (e.g., higher than or equal to 1000° C. and lowerthan or equal to 1500° C.) heat treatment is not performed. In thiscase, the oxide semiconductor film can have a single phase even when thecomposition is slightly out of the range of the single-phase solidsolution range disclosed in Non-Patent Document 1.

When a coordinate point in the solid solution range of theIn:M:Zn=(1+α):(1−α):1 structure is close to Area 12 where the spinelphase is likely to be mixed, instead of a single-phase oxidesemiconductor film (e.g., an oxide semiconductor film in the homologousseries), an oxide semiconductor film including a spinel crystalstructure is formed in some cases.

In the case where the atomic ratio is largely out of the solid solutionrange of the In:M:Zn=(1+α):(1−α):1 structure, i.e., stoichiometry of thesingle phase, a plurality of crystal structures (e.g., two phases orthree phases) are mixed in the oxide semiconductor film in some cases.In the case where a plurality of crystal structures are mixed in anoxide semiconductor film, a grain boundary might be formed betweendifferent crystal structures, so that the electrical characteristics orreliability of the oxide semiconductor film might deteriorate. Moreover,in the case where a plurality of crystal structures are mixed in theoxide semiconductor film, variation in crystallinity of the oxidesemiconductor film might be caused.

Accordingly, the oxide semiconductor film of one embodiment of thepresent invention preferably has a crystal structure exhibiting a singlephase, particularly, homologous series.

When the oxide semiconductor film of one embodiment of the presentinvention has a composition in the neighborhood of the solid solutionrange of the In_(1+x)M_(1−x)O₃(ZnO)_(y) structure (x satisfies 0<x<0.5,and y is approximately 1) and contains a higher proportion of In than M,its composition is far from Area 12 where a spinel phase is likely to bemixed and a single-phase crystal structure is obtained. In addition, theoxide semiconductor film containing In in a higher proportion can havehigh carrier mobility (electron mobility).

In particular, the oxide semiconductor film of one embodiment of thepresent invention preferably has a composition in the neighborhood ofthe solid solution range of the In_(1+x)M_(1−x)O₃(ZnO)_(y) structure (xsatisfies 0<x<0.5, and y is approximately 1), specifically a compositionin the neighborhood of the coordinate point A denoted by the blackcircle in FIG. 1 (In:M:Zn=1.33:0.67:1 (around In:M:Zn=4:2:3)).

In this specification and the like, “neighborhood” means a range of ±1,preferably ±0.5 with respect to the proportion of atoms of the metalelement M. For example, in the case where the oxide semiconductor filmhas a composition in the neighborhood of In:Ga:Zn=4:2:3, the proportionof Ga may be greater than or equal to 1 and less than or equal to 3(1≤Ga≤3) and the proportion of Zn is greater than or equal to 2 and lessthan or equal to 4 (2≤Zn≤4), preferably the proportion of Ga is greaterthan or equal to 1.5 and less than or equal to 2.5 (1.5≤Ga≤2.5) and theproportion of Zn is greater than or equal to 2 and less than or equal to4 (2≤Zn≤4). Moreover, in this specification and the like,“approximately” means a range of ±0.2 with respect to a value. Forexample, “approximately 1” means 0.8 or more and 1.2 or less.

In the case where the oxide semiconductor film is formed with asputtering apparatus, a film having an atomic ratio deviated from theatomic ratio of the target is formed in some cases. Especially for Zn,depending on the substrate temperature during deposition, the proportionof Zn atoms in a deposited film is lower than that of Zn atoms in thetarget in some cases. Therefore, an oxide semiconductor film having anatomic ratio deviated from the atomic ratio of the target is alsoincluded in the category of the oxide semiconductor film of oneembodiment of the present invention. The target used for the sputteringapparatus is preferably a polycrystalline metal oxide target.

Here, results of investigating the relation between the atomic ratio ofa target including In, Ga, and Zn and the atomic ratio of an oxidesemiconductor film obtained by a sputtering method using the target aredescribed. Table 1 shows the atomic ratios of targets used for theinvestigation and the atomic ratios of oxide semiconductor filmsobtained by a sputtering method. Polycrystalline metal oxide targets areused as the targets.

TABLE 1 Target atomic ratio Film atomic ratio In:Ga:Zn In Ga Zn 1:1:1 10.99 0.66   1:1:1.2 1 0.98 0.84   1:1:1.5 1 0.92 1.04 1:3:2 1 2.89 1.241:3:4 1 3.02 2.56 1:3:6 1 3.06 4.19 3:1:2 3 0.98 1.40 3:1:3 3 1.01 2.30

As for the deposition conditions, argon and oxygen were used asdeposition gases and the flow rate percentage of oxygen was 33%. Here,the flow rate percentage of oxygen is expressed by (the flow rate ofoxygen)−((the flow rate of oxygen)+(the flow rate of argon))×100 [%]. Inaddition, the pressure was in the range of 0.4 Pa to 0.7 Pa, thesubstrate temperature was 200° C. to 300° C., and the power supply (DC)was 0.5 kW. A parallel-plate-type sputtering apparatus was used as thedeposition apparatus.

The film atomic ratios in Table 1 were evaluated by an inductivelycoupled plasma mass spectrometry (ICP-MS). In the atomic ratio of thefilm in Table 1, In is normalized as 1 or 3 and Ga and Zn arecalculated.

From the results in Table 1, the values of In and Ga in the atomic ratioof the film are not largely different from those in the target atomicratio. In contrast, the value of Zn in the atomic ratio of the film islargely different from that in the atomic ratio of the target.

The relation between the atomic ratio of the target and the percentageof remaining Zn in the film was determined. FIG. 3 shows the relationbetween the atomic ratio of the target and the percentage of remainingZn. Numbers in FIG. 3 each represent the atomic ratio of In:Ga:Zn of thetarget in Table 1. In FIG. 3 , Zn(Film) on the vertical axis indicates avalue obtained by dividing the proportion of atoms of Zn of a filmobtained by deposition by the sum of the proportions of atoms of In, Ga,and Zn, and Zn(Target) on the vertical axis indicates a value obtainedby dividing the proportion of atoms of Zn of a target by the sum of theproportions of atoms of In, Ga, and Zn of the target. Furthermore,Zn(Film)/Zn(Target)×100 on the vertical axis in FIG. 3 indicates thepercentage of remaining Zn in the oxide semiconductor film. Moreover,Zn(Film)/Ga(Film) on the horizontal axis in FIG. 3 indicates a valueobtained by dividing the proportion of atoms of Zn of the film obtainedby deposition by the proportion of atoms of Ga of the film obtained bythe deposition.

The results in FIG. 3 show a favorable correlation between the atomicratio of Zn to Ga in the film (Zn(Film)/Ga(Film)) and the percentage ofremaining Zn (Zn(Film)/Zn(Target)×100). That is, as the proportion of Zngets lower than that of Ga, the percentage of remaining Zn is decreased.

As shown in FIG. 3 , the percentage of remaining Zn of the oxidesemiconductor film obtained by a sputtering method ranges from greaterthan or equal to 50% and less than or equal to 90% with respect to theatomic ratio of the target. That is, the proportion of Zn of thedeposited oxide semiconductor film is significantly reduced comparedwith that in the atomic ratio of the target in some cases.

For example, in the case where the desired composition of the oxidesemiconductor film is In:Ga:Zn=4:2:3, the composition of the targetneeds to be the composition at Zn(Film)/Ga(Film) of 1.5 on thehorizontal axis in FIG. 3 . Since the percentage of remaining Zn atZn(Film)/Ga(Film) of 1.5 is about 74%, the composition of the target ispreferably in the neighborhood of In:Ga:Zn=4:2:4.1.

Note that the oxide semiconductor film of one embodiment of the presentinvention is preferably CAAC-OS described later. Furthermore, it ispreferable that the CAAC-OS have no spinel crystal structure inparticular. The spinel crystal structure in the oxide semiconductor filmcan be evaluated by X-ray diffraction (XRD) analysis, for example.

<1-2. Structure of Oxide Semiconductor>

Next, a structure of an oxide semiconductor included in the oxidesemiconductor film of one embodiment of the present invention isdescribed.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorthat has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

[Caac-Os]

First, a CAAC-OS will be described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 4A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 4B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 4A. FIG. 4B shows that metal atoms are arranged in alayered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or thetop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 4B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 4C. FIGS. 4B and 4C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc). Furthermore, the CAAC-OS canalso be referred to as an oxide semiconductor including c-axis alignednanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 4D). The part in which the pellets are tilted as observed inFIG. 4C corresponds to a region 5161 shown in FIG. 4D.

FIG. 5A shows a Cs-corrected high-resolution TEM image of a plane of theCAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 5B, 5C, and 5D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 5A,respectively. FIGS. 5B, 5C, and 5D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by XRD is described. For example, when thestructure of a CAAC-OS including an InGaZnO₄ crystal is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 31° as shown in FIG. 6A. This peak is assigned to the (009) planeof the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OShave c-axis alignment, and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak at 2θ of around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray beam is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is attributed to the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 6B, apeak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 6C, six peaks which areassigned to crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof the a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 7A can be obtained. In this diffraction pattern, spots assigned tothe (009) plane of an InGaZnO₄ crystal are included. Thus, the electrondiffraction also indicates that pellets included in the CAAC-OS havec-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 7B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 7B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 7B is considered to be assigned to the(010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. Thesecond ring in FIG. 7B is considered to be assigned to the (110) planeand the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacanciesin the oxide semiconductor serve as carrier traps or serve as carriergeneration sources when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferablylower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Suchan oxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

[nc-OS]

Next, an nc-OS will be described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakindicating a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS, a plurality of spots areshown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

[A-Like OS]

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (referred to as Sample A), an nc-OS (referred to as SampleB), and a CAAC-OS (referred to as Sample C) are prepared as samplessubjected to electron irradiation. Each of the samples is an In—Ga—Znoxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of an InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 8 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 8 indicates that thecrystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 8 , acrystal part of approximately 1.2 nm (also referred to as an initialnucleus) at the start of TEM observation grows to a size ofapproximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². Incontrast, the crystal part size in the nc-OS and the CAAC-OS showslittle change from the start of electron irradiation to a cumulativeelectron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3)in FIG. 8 , the average crystal sizes in an nc-OS and a CAAC-OS areapproximately 1.4 nm and approximately 2.1 nm, respectively, regardlessof the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

<1-3. Formation Method of CAAC-OS Film>

An example of a method for forming a CAAC-OS film will be describedbelow.

FIG. 9A is a schematic view of the inside of a deposition chamber. TheCAAC-OS film can be formed by a sputtering method.

As shown in FIG. 9A, a substrate 5220 and a target 5230 are arranged toface each other. Plasma 5240 is generated between the substrate 5220 andthe target 5230. A heating mechanism 5260 is under the substrate 5220.The target 5230 is attached to a backing plate (not illustrated in thedrawing). A plurality of magnets are arranged to face the target 5230with the backing plate positioned therebetween. A sputtering method inwhich the deposition speed is increased by utilizing a magnetic field ofmagnets is referred to as a magnetron sputtering method.

The distance d between the substrate 5220 and the target 5230 (alsoreferred to as a target-substrate distance (T-S distance)) is greaterthan or equal to 0.01 m and less than or equal to 1 m, preferablygreater than or equal to 0.02 m and less than or equal to 0.5 m. Thedeposition chamber is mostly filled with a deposition gas (e.g., anoxygen gas, an argon gas, or a mixed gas containing oxygen at 5 volume %or higher) and the pressure in the deposition chamber is controlled tobe higher than or equal to 0.01 Pa and lower than or equal to 100 Pa,preferably higher than or equal to 0.1 Pa and lower than or equal to 10Pa. Here, discharge starts by application of a voltage at a certainvalue or higher to the target 5230, and the plasma 5240 can be observed.The magnetic field forms a high-density plasma region in the vicinity ofthe target 5230. In the high-density plasma region, the deposition gasis ionized, so that an ion 5201 is generated. Examples of the ion 5201include an oxygen cation (O+) and an argon cation (Ar⁺).

Here, the target 5230 has a polycrystalline structure which includes aplurality of crystal grains and in which a cleavage plane exists in anyof the crystal grains. FIG. 10 shows a crystal structure of InMZnO₄ (theelement M is Al, Ga, Y, or Sn, for example) included in the target 5230as an example. Note that FIG. 10 illustrates the crystal structure ofInMZnO₄ observed from a direction parallel to the b-axis. In the crystalof InMZnO₄, oxygen atoms are negatively charged, whereby repulsive forceis generated between the two adjacent M-Zn—O layers. Thus, the InMZnO₄crystal has a cleavage plane between the two adjacent M-Zn—O layers.

The ion 5201 generated in the high-density plasma region is acceleratedtoward the target 5230 side by an electric field, and then collides withthe target 5230. At this time, a pellet 5200 which is a flat-plate-likeor pellet-like sputtered particles is separated from the cleavage plane(FIG. 9A). The pellet 5200 is between the two cleavage planes shown inFIG. 10 . Thus, when the pellet 5200 is observed, the cross-sectionthereof is as shown in FIG. 9B, and the top surface thereof is as shownin FIG. 9C. Note that the structure of the pellet 5200 may be distortedby an impact of collision of the ion 5201. Note that along with theseparation of the pellet 5200, a particle 5203 is also sputtered fromthe target 5230. The particle 5203 has an atom or an aggregate ofseveral atoms. Therefore, the particle 5203 can be referred to as anatomic particle.

The pellet 5200 is a flat-plate-like (pellet-like) sputtered particlehaving a triangle plane, e.g., regular triangle plane. Alternatively,the pellet 5200 is a flat-plate-like (pellet-like) sputtered particlehaving a hexagon plane, e.g., regular hexagon plane. However, the shapeof a flat plane of the pellet 5200 is not limited to a triangle or ahexagon. For example, the flat plane may have a shape formed bycombining two or more triangles. For example, a quadrangle (e.g.,rhombus) may be formed by combining two triangles (e.g., regulartriangles).

The thickness of the pellet 5200 is determined depending on the kind ofthe deposition gas and the like. For example, the thickness of thepellet 5200 is greater than or equal to 0.4 nm and less than or equal to1 nm, preferably greater than or equal to 0.6 nm and less than or equalto 0.8 nm. In addition, for example, the width of the pellet 5200 isgreater than or equal to 1 nm and less than or equal to 100 nm,preferably greater than or equal to 2 nm and less than or equal to 50nm, further preferably greater than or equal to 3 nm and less than orequal to 30 nm. For example, the ion 5201 collides with the target 5230including the In-M-Zn oxide. Then, the pellet 5200 including threelayers of an M-Zn—O layer, an In—O layer, and an M-Zn—O layer isseparated.

The pellet 5200 may receive a charge when passing through the plasma5240, so that surfaces thereof are negatively or positively charged. Forexample, the pellet 5200 receives a negative charge from O²⁻ in theplasma 5240. As a result, oxygen atoms on the surfaces of the pellet5200 may be negatively charged. In addition, when passing through theplasma 5240, the pellet 5200 is sometimes combined with indium, theelement M, zinc, oxygen, or the like in the plasma 5240 to grow up.

The pellet 5200 and the particles 5203 that have passed through theplasma 5240 reach a surface of the substrate 5220. Note that some of theparticles 5203 are discharged to the outside by a vacuum pump or thelike because of their smallness in mass.

Next, deposition of the pellet 5200 and the particle 5203 over thesurface of the substrate 5220 is described with reference to FIGS. 11Ato 11F.

First, a first pellet 5200 is deposited over the substrate 5220. Sincethe pellet 5200 has a flat-plate-like shape, it is deposited so that theflat plane faces the surface of the substrate 5220. At this time, acharge on a surface of the pellet 5200 on the substrate 5220 side islost through the substrate 5220.

Next, a second pellet 5200 reaches the substrate 5220. Since a surfaceof the first pellet 5200 and a surface of the second pellet 5200 arecharged, they repel each other. As a result, the second pellet 5200avoids being deposited over the first pellet 5200, and is deposited withits flat plane facing the surface of the substrate 5220 so as to be alittle distance away from the first pellet 5200. With repetition ofthis, millions of the pellets 5200 are deposited on the surface of thesubstrate 5220 to have a thickness of one layer. A region where nopellet 5200 is deposited is generated between adjacent pellets 5200 (seeFIG. 11A).

Then, the particles 5203 that have received energy from plasma reach thesurface of the substrate 5220. The particles 5203 cannot be deposited onan active region such as the surfaces of the pellets 5200. For thisreason, the particles 5203 move to regions where no pellet 5200 isdeposited and are attached to side surfaces of the pellets 5200. Sinceavailable bonds of the particles 5203 are activated by energy receivedfrom plasma, the particles 5203 are chemically bonded to the pellets5200 to form lateral growth portions 5202 (see FIG. 11B).

The lateral growth portions 5202 then further grow laterally so that thepellets 5200 are anchored to each other (see FIG. 11C). In this manner,the lateral growth portions 5202 are formed until they fill regionswhere no pellet 5200 is deposited. This mechanism is similar to adeposition mechanism for an atomic layer deposition (ALD) method.

Even when the deposited pellets 5200 are oriented in differentdirections, the particles 5203 cause a lateral growth to fill gapsbetween the pellets 5200; thus, no clear grain boundary is formed. Inaddition, as the particles 5203 make a smooth connection between thepellets 5200, a crystal structure different from single crystal andpolycrystal structures is formed. In other words, a crystal structureincluding distortion between minute crystal regions (pellets 5200) isformed. Regions filling the gaps between the crystal regions aredistorted crystal regions, and thus, it will be not appropriate to saythat the regions have an amorphous structure.

Next, new pellets 5200 are deposited with their flat planes facing thesurface of the substrate 5220 (see FIG. 11D). After that, the particles5203 are deposited so as to fill regions where no pellet 5200 isdeposited, thereby forming the lateral growth portions 5202 (see FIG.11E). In such a manner, the particles 5203 are attached to side surfacesof the pellets 5200 and the lateral growth portions 5202 cause a lateralgrowth so that the pellets 5200 in the second layer are anchored to eachother (see FIG. 11F). Deposition continues until the m-th layer (m is aninteger of two or more) is formed; as a result, a stacked-layer thinfilm structure is formed.

A deposition way of the pellets 5200 changes according to the surfacetemperature of the substrate 5220 or the like. For example, if thesurface temperature of the substrate 5220 is high, migration of thepellets 5200 occurs over the surface of the substrate 5220. As a result,a proportion of the pellets 5200 that are directly connected with eachother without the particles 5203 increases, whereby a CAAC-OS with highorientation is made. The surface temperature of the substrate 5220 forformation of the CAAC-OS is higher than or equal to room temperature andlower than 340° C., preferably higher than or equal to room temperatureand lower than or equal to 300° C., further preferably higher than orequal to 100° C. and lower than or equal to 250° C., still furtherpreferably higher than or equal to 100° C. and lower than or equal to200° C. Therefore, even when a large-sized substrate of the 8thgeneration or more is used as the substrate 5220, a warp or the like dueto the deposition of the CAAC-OS hardly occurs.

In contrast, if the surface temperature of the substrate 5220 is low,the migration of the pellets 5200 over the substrate 5220 does noteasily occur. As a result, the pellets 5200 are stacked to form an nc-OSor the like with low orientation. In the nc-OS, the pellets 5200 arepossibly deposited with certain gaps because the pellets 5200 arenegatively charged. Therefore, the nc-OS has low orientation but someregularity, and thus it has a denser structure than an amorphous oxidesemiconductor.

When spaces between pellets are extremely small in a CAAC-OS, thepellets may form a large pellet. The inside of the large pellet has asingle crystal structure. For example, the size of the pellet may begreater than or equal to 10 nm and less than or equal to 200 nm, greaterthan or equal to 15 nm and less than or equal to 100 nm, or greater thanor equal to 20 nm and less than or equal to 50 nm, when seen from theabove.

The pellets are considered to be deposited on the surface of thesubstrate according to such a deposition model. A CAAC-OS can bedeposited even when a formation surface does not have a crystalstructure. This indicates that the above-described deposition model,which is a growth mechanism different from an epitaxial growth, has highvalidity. In addition, with the above-described deposition model, auniform film of a CAAC-OS or an nc-OS can be formed even over alarge-sized glass substrate or the like. Even when the surface of thesubstrate (formation surface) has an amorphous structure (e.g.,amorphous silicon oxide), for example, a CAAC-OS can be formed.

In addition, even when the surface of the substrate (formation surface)has an uneven shape, the pellets are aligned along the shape.

The above-described deposition model suggests that a CAAC-OS with highcrystallinity can be formed in the following manner: deposition isperformed in a high vacuum to have a long mean free path, plasma energyis weakened to reduce damage around a substrate, and thermal energy isapplied to a formation surface to repair damage due to plasma duringdeposition.

The above-described deposition model can be used not only for the casewhere a target has a polycrystalline structure of a composite oxide witha plurality of crystal grains, such as an In-M-Zn oxide, and any of thecrystal grains have a cleavage plane; but also for the case where, forexample, a target of a mixture containing indium oxide, an oxide of theelement M, and zinc oxide is used.

Since there is no cleavage plane in a target of a mixture, atomicparticles are separated from the target by sputtering. Duringdeposition, a high electric field region of plasma is formed around atarget. Because of the high electric field region of plasma, atomicparticles separated from the target are anchored to each other to causea lateral growth. For example, indium atoms, which are atomic particles,are anchored to each other and cause a lateral growth to be ananocrystal formed of an In—O layer, and then an M-Zn—O layer is bondedabove and below the nanocrystalline In—O layer so as to complement thenanocrystalline In—O layer. In this manner, a pellet can be formed evenwhen a target of a mixture is used. Accordingly, the above-describeddeposition model can also be applied to the case of using a target of amixture.

Note that in the case where a high electric field region of plasma isnot formed around a target, only atomic particles separated from thetargets are deposited on a substrate surface. In that case, a lateralgrowth of an atomic particle might occur on the substrate surface.However, since the orientations of atomic particles are not the same,the crystal orientation in the resulting thin film is not uniform. As aresult, an nc-OS or the like is obtained.

<1-4. Lateral Growth>

The following description explains that a lateral growth occurs when theparticles 5203 are attached to (bonded to or adsorbed on) the pellet5200 laterally.

FIGS. 12A to 12E illustrate a structure of the pellet 5200 and positionsto which metal ions can be attached. A model assumed as the pellet 5200is a cluster model with 84 atoms extracted from an InMZnO₄ crystalstructure with a constant stoichiometric composition. The case where theelement M is Ga is described below. FIG. 12F illustrates a structure ofthe pellet 5200 seen in the direction parallel to the c-axis. FIG. 12Gillustrates a structure of the pellet 5200 seen in the directionparallel to the a-axis.

The positions to which metal ions can be attached are represented as aposition A, a position B, a position a, a position b, and a position c.The position A is an upper part of an interstitial site surrounded byone gallium atom and two zinc atoms on the top surface of the pellet5200. The position B is an upper part of an interstitial site surroundedby two gallium atoms and one zinc atom on the top surface of the pellet5200. The position a is in an indium site on a side surface of thepellet 5200. The position b is in an interstitial site between an In—Olayer and a Ga—Zn—O layer on a side surface of the pellet 5200. Theposition c is in a gallium site on a side surface of the pellet 5200.

The relative energy was estimated from first principles calculation ineach case where a metal ion was located in the assumed position (theposition A, the position B, the position a, the position b, or theposition c). In the calculation, first principles calculation softwareVASP (Vienna Ab initio Simulation Package) was used. For theexchange-correlation potential, Perdew-Burke-Ernzerhof (PBE) typegeneralized gradient approximation (GGA) was used, and for the ionpotential, a projector augmented wave (PAW) method was used. The cut-offenergy was 400 eV, and Γ-only k-point sampling was used. Table 2 showsthe relative energies in the case where an indium ion (In³⁺), a galliumion (Ga³⁺), and a zinc ion (Zn²⁺) are located at the position A, theposition B, the position a, the position b, and the position c. Notethat the relative energy is a relative value under the condition wherethe energy of the model with the lowest energy among the calculatedmodels is set to 0 eV.

TABLE 2 Relative Energy [eV] Top surface of pellet Side surface ofpellet Ion A B a b c In³⁺ 2.1 1.5 0.0 1.8 1.9 Ga³⁺ 3.7 3.0 0.6 0.0 3.5Zn²⁺ 2.3 1.8 0.0 0.6 2.9

It is found that any metal ion is more likely to be attached to the sidesurface of the pellet 5200 than to the top surface thereof. It is alsofound that a zinc ion as well as an indium ion is most likely to beattached to the indium site at the position a.

Ease of an oxygen ion (O²⁺) attached to the pellet 5200 was examined.FIGS. 13A to 13E illustrate a structure of the pellet 5200 and positionsto which oxygen ions can be attached. FIG. 13F illustrates a structureof the pellet 5200 seen in the direction parallel to the c-axis. FIG.13G illustrates a structure of the pellet 5200 seen in the directionparallel to the b-axis.

The positions to which oxygen ions can be attached are represented as aposition C, a position D, a position d, a position e, and a position fIn the position C, an oxygen ion is bonded to gallium on the top surfaceof the pellet 5200. In the position D, an oxygen ion is bonded to zincon the top surface of the pellet 5200. In the position d, an oxygen ionis bonded to indium on a side surface of the pellet 5200. In theposition e, an oxygen ion is bonded to gallium on a side surface of thepellet 5200. In the position f, an oxygen ion is bonded to zinc on aside surface of the pellet 5200.

The relative energy was estimated from first principles calculation ineach case where an oxygen ion was located in the assumed position (theposition C, the position D, the position d, the position e, or theposition f). Table 3 shows the relative energies in the case whereoxygen ions (O²⁻) are located at the position C, the position D, theposition d, the position e, and the position f.

TABLE 3 Relative Energy [eV] Top surface of pellet Side surface ofpellet Ion C D d e f O²⁻ 3.9 3.6 0.0 0.5 1.5

It is found that the oxygen ion is also likely to be attached to theside surface of the pellet 5200 than to the top surface thereof.

According to the above, the particle 5203 that has approached the pellet5200 is preferentially attached to the side surface of the pellet 5200.This suggests that the deposition model in which a lateral growth of thepellet 5200 occurs when the particles 5203 are attached to the sidesurface of the pellet 5200 has high validity.

The pellets 5200 are considered to be deposited on the surface of thesubstrate 5220 according to such a deposition model. A CAAC-OS can bedeposited even when a formation surface does not have a crystalstructure; therefore, a growth mechanism in this case is different fromepitaxial growth. In addition, a uniform film of a CAAC-OS or an nc-OScan be formed even over a large-sized glass substrate or the like. Evenwhen the surface of the substrate 5220 (formation surface) has anamorphous structure (e.g., amorphous silicon oxide), for example, aCAAC-OS can be formed.

In addition, even when the surface of the substrate 5220 (formationsurface) has an uneven shape, the pellets 5200 are aligned along theshape.

According to one embodiment of the present invention, an oxidesemiconductor film having no or a very small region in which a spinelcrystal structure is observed can be obtained. A highly reliablesemiconductor device which includes the oxide semiconductor film can beprovided.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments and the examples.

Embodiment 2

In this embodiment, semiconductor devices including the oxidesemiconductor film of one embodiment of the present invention, andmanufacturing methods thereof will be described with reference to FIGS.14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS. 17A to 17C, FIGS.18A to 18D, FIGS. 19A and 19B, FIGS. 20A to 20D, FIGS. 21A to 21F, FIGS.22A to 22F, FIGS. 23A to 23F, FIGS. 24A and 24B, FIGS. 25A to 25F, andFIGS. 26A to 26F.

<2-1. Structural Example of Semiconductor Device>

FIG. 14A is a plan view of a transistor 200 that is a semiconductordevice of one embodiment of the present invention. FIG. 14B is across-sectional view taken along a dashed dotted line X1-X2 in FIG. 14A,and FIG. 14C is a cross-sectional view taken along a dashed dotted lineY1-Y2 in FIG. 14A. Note that in FIG. 14A, some components of thetransistor 200 (e.g., an insulating film serving as a gate insulatingfilm) are not illustrated to avoid complexity. Furthermore, thedirection of the dashed dotted line X1-X2 may be referred to as achannel length direction, and the direction of the dashed dotted lineY1-Y2 may be referred to as a channel width direction. As in FIG. 14A,some components are not illustrated in some cases in top views oftransistors described below.

The transistor 200 includes a conductive film 204 functioning as a gateelectrode over a substrate 202, an insulating film 206 over thesubstrate 202 and the conductive film 204, an insulating film 207 overthe insulating film 206, an oxide semiconductor film 208 over theinsulating film 207, a conductive film 212 a functioning as a sourceelectrode electrically connected to the oxide semiconductor film 208,and a conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208. Over thetransistor 200, specifically, over the conductive films 212 a and 212 band the oxide semiconductor film 208, an insulating film 214, aninsulating film 216, and an insulating film 218 are provided. Theinsulating films 214, 216, and 218 function as protective insulatingfilms for the transistor 200.

Furthermore, the insulating films 206 and 207 function as gateinsulating films of the transistor 200.

As the oxide semiconductor film 208, the oxide semiconductor filmdescribed in Embodiment 1 can be used. Since the oxide semiconductorfilm of one embodiment of the present invention has no or very fewregions with a spinel crystal structure, the highly reliable transistor200 can be provided.

Components of the semiconductor device of this embodiment will bedescribed below in detail.

[Substrate]

There is no particular limitation on the property of a material and thelike of the substrate 202 as long as the material has heat resistancehigh enough to withstand at least heat treatment to be performed later.For example, a glass substrate, a ceramic substrate, a quartz substrate,a sapphire substrate, or the like may be used as the substrate 202.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI (silicon on insulator) substrate, or thelike may be used as the substrate 202. Further alternatively, any ofthese substrates provided with a semiconductor element may be used asthe substrate 202. In the case where a glass substrate is used as thesubstrate 202, a glass substrate having any of the following sizes canbe used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation(2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, alarge-sized display device can be manufactured. Such a large-sizedsubstrate is preferably used because the manufacturing cost can bereduced.

Alternatively, a flexible substrate may be used as the substrate 202,and the transistor 200 may be provided directly on the flexiblesubstrate. Further alternatively, a separation layer may be providedbetween the substrate 202 and the transistor 200. The separation layercan be used when part or the whole of a semiconductor device formed overthe separation layer is completed and separated from the substrate 202and transferred to another substrate. In such a case, the transistor 200can be transferred to a substrate having low heat resistance or aflexible substrate as well.

[Conductive films functioning as gate electrode and source and drainelectrodes] The conductive film 204 functioning as a gate electrode, theconductive film 212 a functioning as a source electrode, and theconductive film 212 b functioning as a drain electrode can each beformed using a metal element selected from chromium (Cr), copper (Cu),aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo),tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni),iron (Fe), and cobalt (Co); an alloy including any of these metalelements as its component; an alloy including a combination of any ofthese metal elements; or the like.

The conductive films 204, 212 a, and 212 b may have a single-layerstructure or a stacked-layer structure of two or more layers. Forexample, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a titanium nitride film, a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, an alloy film or anitride film in which aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium are combined may be used.

The conductive films 204, 212 a, and 212 b can also be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium zinc oxide, or indium tin oxide towhich silicon oxide is added.

A Cu X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive films 204, 212 a, and 212 b. Use of a Cu X alloy filmenables the manufacturing cost to be reduced because wet etching processcan be used in the processing.

[Insulating Film Functioning as Gate Insulating Film]

As each of the insulating films 206 and 207 functioning as gateinsulating films of the transistor 200, an insulating layer including atleast one of the following films formed by a plasma-enhanced chemicalvapor deposition (PECVD) method, a sputtering method, or the like can beused: a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, an yttrium oxide film, a zirconium oxide film, a galliumoxide film, a tantalum oxide film, a magnesium oxide film, a lanthanumoxide film, a cerium oxide film, and a neodymium oxide film. Note thatinstead of the stacked-layer structure of the insulating films 206 and207, an insulating film of a single layer formed using a materialselected from the above or an insulating film of three or more layersmay be used.

The insulating film 206 functions as a blocking film which inhibitspenetration of oxygen. For example, in the case where excess oxygen issupplied to the insulating film 207, the insulating film 214, theinsulating film 216, and/or the oxide semiconductor film 208, theinsulating film 206 can inhibit penetration of oxygen.

Note that the insulating film 207 that is in contact with the oxidesemiconductor film 208 functioning as a channel region of the transistor200 is preferably an oxide insulating film and preferably includes aregion including oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 207 is aninsulating film capable of releasing oxygen. In order to provide theoxygen-excess region in the insulating film 207, the insulating film 207is formed in an oxygen atmosphere, for example. Alternatively, theoxygen-excess region may be formed by introduction of oxygen into theinsulating film 207 after the deposition. As a method for introducingoxygen, an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like may beemployed.

In the case where hafnium oxide is used as the insulating film 207, thefollowing effect is attained. Hafnium oxide has a higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, by usinghafnium oxide, the thickness of the insulating film 207 can be madelarge as compared with the case where silicon oxide is used; thus,leakage current due to tunnel current can be low. That is, it ispossible to provide a transistor with a low off-state current. Moreover,hafnium oxide with a crystalline structure has higher dielectricconstant than hafnium oxide with an amorphous structure. Therefore, itis preferable to use hafnium oxide with a crystalline structure in orderto provide a transistor with a low off-state current. Examples of thecrystalline structure include a monoclinic crystal structure and a cubiccrystal structure. Note that one embodiment of the present invention isnot limited thereto.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 206, and a silicon oxide film is formed as the insulating film 207.The silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for capacitance equivalent tothat of the silicon oxide film. Thus, when the silicon nitride film isincluded as the gate insulating film of the transistor 200, thethickness of the insulating film can be physically increased. This makesit possible to reduce a decrease in withstand voltage of the transistor200 and furthermore to increase the withstand voltage, thereby reducingelectrostatic discharge damage to the transistor 200.

[Oxide Semiconductor Film]

As the oxide semiconductor film 208, the oxide semiconductor film of oneembodiment of the present invention described in Embodiment 1 can beused.

The energy gap of the oxide semiconductor film 208 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. The use ofan oxide semiconductor having such a wide energy gap can reduceoff-state current of the transistor 200.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 208. For example, an oxide semiconductor filmwhose carrier density is lower than 8×10¹¹/cm³, preferably lower than1×10¹¹/cm³, or further preferably lower than 1×10¹⁰/cm³, and greaterthan or equal to 1×10⁻⁹/cm³ is used as the oxide semiconductor film 208.

Note that, without limitation to the compositions and materialsdescribed above, a material with an appropriate composition may be useddepending on required semiconductor characteristics and electricalcharacteristics (e.g., field-effect mobility and threshold voltage) of atransistor. Furthermore, in order to obtain required semiconductorcharacteristics of a transistor, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio of a metal element to oxygen, the interatomic distance, thedensity, and the like of the oxide semiconductor film 208 be set to beappropriate.

Note that it is preferable to use, as the oxide semiconductor film 208,an oxide semiconductor film in which the impurity concentration is lowand the density of defect states is low, in which case the transistorcan have more excellent electrical characteristics. Here, the state inwhich the impurity concentration is low and the density of defect statesis low (the number of oxygen vacancies is small) is referred to as“highly purified intrinsic” or “substantially highly purifiedintrinsic”. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor film has few carrier generation sources,and thus can have a low carrier density. Thus, a transistor in which achannel region is formed in the oxide semiconductor film rarely has anegative threshold voltage (is rarely normally on). A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has a low density of defect states and accordingly has a lowdensity of trap states in some cases. Furthermore, the highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has an extremely low off-state current; even when an element has achannel width W of 1×10⁶ μm and a channel length L of 10 μm, theoff-state current can be less than or equal to the measurement limit ofa semiconductor parameter analyzer, that is, less than or equal to1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and adrain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film can have a small change in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor whose channel region isformed in the oxide semiconductor film having a high density of trapstates has unstable electrical characteristics in some cases. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, alkalineearth metal, iron, nickel, silicon, and the like are given.

In the case where the oxide semiconductor film 208 includes In, Ga, andZn, for example, the highly purified intrinsic oxide semiconductor film208 preferably includes a region where the total amount of In, Ga, Zn,and a is higher than or equal to 99.97 atomic %.

Hydrogen included in the oxide semiconductor film 208 reacts with oxygenbonded to a metal atom to be water, and also causes oxygen vacancies ina lattice from which oxygen is released (or a portion from which oxygenis released). Due to entry of hydrogen into the oxygen vacancies,electrons serving as carriers are generated in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film which contains hydrogenis likely to be normally on. Accordingly, it is preferable that hydrogenbe reduced as much as possible in the oxide semiconductor film 208.Specifically, the hydrogen concentration of the oxide semiconductor film208, which is measured by secondary ion mass spectrometry (SIMS), islower than 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³,further preferably lower than 5×10¹⁸ atoms/cm³, still further preferablylower than 1×10¹⁸ atoms/cm³. In other words, the oxide semiconductorfilm 208 includes a region whose hydrogen concentration measured by SIMSis lower than 1×10²⁰ atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 208, oxygen vacancies areincreased in the oxide semiconductor film 208, and the oxidesemiconductor film 208 becomes an n-type film. Thus, the concentrationof silicon or carbon (the concentration is measured by SIMS) in theoxide semiconductor film 208 or the concentration of silicon or carbon(the concentration is measured by SIMS) in the vicinity of an interfacewith the oxide semiconductor film 208 is set to be lower than or equalto 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

When the oxide semiconductor film 208 contains alkali metal or alkalineearth metal, the alkali metal or the alkaline earth metal and oxygen orthe like contained in the oxide semiconductor film are bonded to eachother, so that carriers are generated in some cases. Thus, a transistorincluding an oxide semiconductor film which contains alkali metal oralkaline earth metal is likely to be normally on. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 208. Specifically, theconcentration of alkali metal or alkaline earth metal of the oxidesemiconductor film 208, which is measured by SIMS, is lower than orequal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶atoms/cm³.

In some cases, iron, nickel, and silicon contained in the oxidesemiconductor film 208 are bonded to oxygen or the like contained in theoxide semiconductor film, so that carriers are generated. Thus, atransistor including an oxide semiconductor film which contains iron,nickel, and silicon is likely to be normally on. Therefore, it ispreferable to reduce the concentrations of iron, nickel, and silicon ofthe oxide semiconductor film 208 are preferably reduced. For example,the total concentration of impurities, i.e., iron, nickel, and siliconin the oxide semiconductor film 208 may be lower than 0.03 atomic %.

When the oxide semiconductor film 208 contains nitrogen, the oxidesemiconductor film 208 easily becomes n-type by generation of electronsserving as carriers and an increase of carrier density. A transistorincluding an oxide semiconductor film which contains nitrogen is likelyto be normally-on. For this reason, nitrogen in the oxide semiconductorfilm is preferably reduced as much as possible; the concentration ofnitrogen which is measured by SIMS is preferably set to be, for example,lower than or equal to 5×10¹⁸ atoms/cm³.

[Insulating Film Functioning as Protective Insulating Film of aTransistor]

The insulating films 214 and 216 each have a function of supplyingoxygen to the oxide semiconductor film 208. The insulating film 218functions as a protective insulating film for the transistor 200. Theinsulating films 214 and 216 contain oxygen. Furthermore, the insulatingfilm 214 is an insulating film which is permeable to oxygen. Note thatthe insulating film 214 serves also as a film which relieves damage tothe oxide semiconductor film 208 at the time of forming the insulatingfilm 216 later.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating film 214.

In addition, it is preferable that the number of defects in theinsulating film 214 be small; as a typical example, the spin densitycorresponding to a signal that appears at around g=2.001 due to adangling bond of silicon be lower than or equal to 3×10¹′ spins/cm³ byelectron spin resonance (ESR) measurement. This is because if thedensity of defects in the insulating film 214 is high, oxygen is bondedto the defects and the amount of oxygen that passes through theinsulating film 214 is decreased.

Note that not all oxygen entering the insulating film 214 from theoutside move to the outside of the insulating film 214 and some oxygenremains in the insulating film 214. Furthermore, movement of oxygenoccurs in the insulating film 214 in some cases in such a manner thatoxygen enters the insulating film 214 and oxygen contained in theinsulating film 214 moves to the outside of the insulating film 214.When an oxide insulating film that allows oxygen to pass through theinsulating film 216 provided over the insulating film 214 can be movedto the oxide semiconductor film 208 through the insulating film 214.

The insulating film 214 can be formed using an oxide insulating filmhaving a low density of states due to nitrogen oxide. Note that thedensity of states due to nitrogen oxide can be formed between thevalence band maximum (E_(v_os)) and the conduction band minimum(E_(v_os)) of the oxide semiconductor film. A silicon oxynitride filmthat releases less nitrogen oxide, an aluminum oxynitride film thatreleases less nitrogen oxide, or the like can be used as the above oxideinsulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film which releases ammonia more than nitrogen oxide in thermaldesorption spectroscopy analysis; as a typical example, the amount ofreleased ammonia molecules is greater than or equal to 1×10¹⁸molecules/cm³ and less than or equal to 5×10¹⁹ molecules/cm³. Note thatthe amount of released ammonia is the amount of ammonia released by heattreatment with which the surface temperature of the film becomes atemperature higher than or equal to 50° C. and lower than or equal to650° C., preferably higher than or equal to 50° C. and lower than orequal to 550° C.

Nitrogen oxide (NO_(x); x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2),typified by NO₂ or NO, forms levels in the insulating film 214, forexample. The level is positioned in the energy gap of the oxidesemiconductor film 208. Therefore, when nitrogen oxide is diffused tothe interface between the insulating film 214 and the oxidesemiconductor film 208, an electron is in some cases trapped by thelevel on the insulating film 214 side. As a result, the trapped electronremains in the vicinity of the interface between the insulating film 214and the oxide semiconductor film 208; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating film 214 reacts with ammoniacontained in the insulating film 216 in heat treatment, nitrogen oxidecontained in the insulating film 214 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 214 andthe oxide semiconductor film 208.

With such an oxide insulating film, the insulating film 214 can reducethe shift in the threshold voltage of the transistor, which leads to asmaller change in the electrical characteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film214 subjected to heat treatment of a manufacturing process of thetransistor, typically, heat treatment at a temperature lower than 400°C. or lower than 375° C. (preferably higher than or equal to 340° C. andlower than or equal to 360° C.), a first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, a second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and a third signal thatappears at a g-factor of greater than or equal to 1.964 and less than orequal to 1.966 are observed. The split width of the first and secondsignals and the split width of the second and third signals that areobtained by ESR measurement using an X-band are each approximately 5 mT.The sum of the spin densities of the first signal that appears at ag-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higherthan or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x), x is greater than 0 and less than or equal to 2,preferably greater than or equal to 1 and less than or equal to 2).Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the smaller theamount of nitrogen oxide in the oxide insulating film is.

The concentration of nitrogen of the above oxide insulating filmmeasured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above oxide insulating film is formed by a PECVD method at asubstrate temperature higher than or equal to 220° C. and lower than orequal to 350° C. using silane and dinitrogen monoxide, whereby a denseand hard film can be formed.

The insulating film 216 is formed using an oxide insulating film thatcontains oxygen at a higher proportion than oxygen in the stoichiometriccomposition. Part of oxygen is released by heating from the oxideinsulating film containing more oxygen than that in the stoichiometriccomposition. The oxide insulating film containing oxygen in excess ofthat in the stoichiometric composition is an oxide insulating film inwhich the amount of released oxygen converted into oxygen atoms isgreater than or equal to 1.0×10¹⁹ atoms/cm², preferably greater than orequal to 3.0×10²⁰ atoms/cm² in thermal desorption spectroscopy (TDS).Note that the surface temperature of a film in the TDS is preferablyhigher than or equal to 100° C. and lower than or equal to 700° C., orhigher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the insulating film 216.

It is preferable that the amount of defects in the insulating film 216be small; as a typical example, the spin density corresponding to asignal which appears at g=2.001 due to a dangling bond of silicon belower than 1.5×10¹⁸ spins/cm³, further preferably lower than or equal to1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 216is provided more apart from the oxide semiconductor film 208 than theinsulating film 214 is; thus, the insulating film 216 may have higherdefect density than the insulating film 214.

Furthermore, the insulating films 214 and 216 can be formed usinginsulating films formed of the same kinds of materials; thus, a boundarybetween the insulating films 214 and 216 cannot be clearly observed insome cases. Thus, in this embodiment, the boundary between theinsulating films 214 and 216 is shown by a dashed line. Although atwo-layer structure of the insulating films 214 and 216 is described inthis embodiment, the present invention is not limited to this structure.For example, a single-layer structure of either one of the insulatingfilms 214 and 216 may be employed.

The insulating film 218 has a function of blocking oxygen, hydrogen,water, alkali metal, alkaline earth metal, or the like. It is possibleto prevent outward diffusion of oxygen from the oxide semiconductor film208, outward diffusion of oxygen included in the insulating films 214and 216, and entry of hydrogen, water, or the like into the oxidesemiconductor film 208 from the outside by providing the insulating film218.

As the insulating film 218, a nitride insulating film can be used, forexample. The nitride insulating film is formed using silicon nitride,silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or thelike. Note that instead of the nitride insulating film having a blockingeffect against oxygen, hydrogen, water, an alkali metal, an alkalineearth metal, and the like, an oxide insulating film having a blockingeffect against oxygen, hydrogen, water, and the like may be provided. Asthe oxide insulating film having a blocking effect against oxygen,hydrogen, water, and the like, an aluminum oxide film, an aluminumoxynitride film, a gallium oxide film, a gallium oxynitride film, anyttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, ahafnium oxynitride film, and the like can be given.

<2-2. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 200 in FIGS.14A to 14C is described with reference to FIGS. 15A to 15C.

FIG. 15A is a top view of a transistor 250 that is a semiconductordevice of one embodiment of the present invention. FIG. 15B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 15A,and FIG. 15C is a cross-sectional view taken along dashed-dotted lineY1-Y2 in FIG. 15A.

The transistor 250 includes the conductive film 204 functioning as agate electrode over the substrate 202, the insulating film 206 over thesubstrate 202 and the conductive film 204, the insulating film 207 overthe insulating film 206, the oxide semiconductor film 208 over theinsulating film 207, the insulating films 214 and 216 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208 through an opening 251 a provided in the insulating films 214 and216, and the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208 through anopening 251 b provided in the insulating films 214 and 216. Over thetransistor 250, specifically over the conductive films 212 a and 212 band the insulating film 216, the insulating film 218 is provided. Theinsulating films 214 and 216 function as protective insulating films forthe oxide semiconductor film 208. The insulating film 218 functions as aprotective insulating film for the transistor 250.

Although the transistor 200 has a channel-etched structure, thetransistor 250 in FIGS. 15A to 15C has a channel-protective structure.Thus, either the channel-etched structure or the channel-protectivestructure can be applied to the semiconductor device of one embodimentof the present invention. The other structures are the same as those ofthe transistor 200 and a similar effect can be obtained.

<2-3. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 250 in FIGS.15A to 15C is described with reference to FIGS. 16A to 16C.

FIG. 16A is a top view of a transistor 260 that is a semiconductordevice of one embodiment of the present invention. FIG. 16B is across-sectional view taken along dashed-dotted line X1-X2 illustrated inFIG. 16A, and FIG. 16C is a cross-sectional view taken alongdashed-dotted line Y1-Y2 illustrated in FIG. 16A.

The transistor 260 includes the conductive film 204 functioning as agate electrode over the substrate 202, the insulating film 206 over thesubstrate 202 and the conductive film 204, the insulating film 207 overthe insulating film 206, the oxide semiconductor film 208 over theinsulating film 207, the insulating films 214 and 216 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208, and the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208. Over thetransistor 260, specifically over the conductive films 212 a and 212 band the insulating film 216, the insulating film 218 is provided. Theinsulating films 214 and 216 function as protective insulating films forthe oxide semiconductor film 208. The insulating film 218 functions as aprotective insulating film for the transistor 260.

The transistor 260 is different from the transistor 250 illustrated inFIGS. 15A to 15C in the shape of the insulating films 214 and 216.Specifically, the insulating films 214 and 216 of the transistor 260have an island shape over a channel region of the oxide semiconductorfilm 208. The other components are the same as those of the transistor250, and a similar effect is obtained.

<2-4. Structural Example of Semiconductor Device>

A structure example different from that of the transistor 200 in FIGS.14A to 14C is described with reference to FIGS. 17A to 17C.

FIG. 17A is a top view of a transistor 270 that is a semiconductordevice of one embodiment of the present invention. FIG. 17B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 17A,and FIG. 17C is a cross-sectional view taken along dashed-dotted lineY1-Y2 in FIG. 17A.

The transistor 270 includes the conductive film 204 functioning as afirst gate electrode over the substrate 202, the insulating film 206over the substrate 202 and the conductive film 204, the insulating film207 over the insulating film 206, the oxide semiconductor film 208 overthe insulating film 207, the insulating films 214 and 216 over the oxidesemiconductor film 208, the conductive film 212 a functioning as asource electrode electrically connected to the oxide semiconductor film208, the conductive film 212 b functioning as a drain electrodeelectrically connected to the oxide semiconductor film 208, theinsulating film 218 over the conductive films 212 a and 212 b and theinsulating film 216, and conductive films 220 a and 220 b over theinsulating film 218.

In the transistor 270, the insulating films 214, 216, and 218 functionas second gate insulating films of the transistor 270. Furthermore, theconductive film 220 a in the transistor 270 functions as, for example, apixel electrode used for a display device. The conductive film 220 a isconnected to the conductive film 212 b through an opening 252 c providedin the insulating films 214, 216, and 218. The conductive film 220 b inthe transistor 270 functions as a second gate electrode (also referredto as a back gate electrode).

As illustrated in FIG. 17C, the conductive film 220 b is connected tothe conductive film 204 functioning as a first gate electrode throughopenings 252 a and 252 b provided in the insulating films 206, 207, 214,216, and 218. Accordingly, the conductive film 220 b and the conductivefilm 204 are supplied with the same potential.

Note that although the structure in which the openings 252 a and 252 bare provided so that the conductive film 220 b and the conductive film204 are connected to each other is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,a structure in which only one of the openings 252 a and 252 b isprovided so that the conductive film 220 b and the conductive film 204are connected to each other, or a structure in which the openings 252 aand 252 b are not provided and the conductive film 220 b and theconductive film 204 are not connected to each other may be employed.Note that in the case where the conductive film 220 b and the conductivefilm 204 are not connected to each other, it is possible to applydifferent potentials to the conductive film 220 b and the conductivefilm 204.

As illustrated in FIG. 17B, the oxide semiconductor film 208 ispositioned to face each of the conductive film 204 functioning as afirst gate electrode and the conductive film 220 b functioning as asecond gate electrode, and is sandwiched between the two conductivefilms functioning as gate electrodes. The lengths in the channel lengthdirection and the channel width direction of the conductive film 220 bfunctioning as a second gate electrode are longer than those in thechannel length direction and the channel width direction of the oxidesemiconductor film 208. The whole oxide semiconductor film 208 iscovered with the conductive film 220 b with the insulating films 214,216, and 218 positioned therebetween. Since the conductive film 220 bfunctioning as a second gate electrode is connected to the conductivefilm 204 functioning as a first gate electrode through the openings 252a and 252 b provided in the insulating films 206 and 207 and theinsulating films 214, 216, and 218, a side surface of the oxidesemiconductor film 208 in the channel width direction faces theconductive film 220 b functioning as a second gate electrode with theinsulating films 214, 216, and 218 positioned therebetween.

In other words, in the channel width direction of the transistor 270,the conductive film 204 functioning as a first gate electrode and theconductive film 220 b functioning as a second gate electrode areconnected to each other through the openings provided in the insulatingfilms 206 and 207 functioning as first gate insulating films and theinsulating films 214, 216, and 218 functioning as second gate insulatingfilms; and the conductive film 204 and the conductive film 220 bsurround the oxide semiconductor film 208 with the insulating films 206and 207 functioning as first gate insulating films and the insulatingfilms 214, 216, and 218 functioning as second gate insulating filmspositioned therebetween.

Such a structure enables the oxide semiconductor film 208 included inthe transistor 270 to be electrically surrounded by electric fields ofthe conductive film 204 functioning as a first gate electrode and theconductive film 220 b functioning as a second gate electrode. A devicestructure of a transistor, like that of the transistor 270, in whichelectric fields of a first gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film where a channel regionis formed can be referred to as a surrounded channel (s-channel)structure.

Since the transistor 270 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 208 by the conductive film 204 functioning as a firstgate electrode; therefore, the current drive capability of thetransistor 270 can be improved and high on-state current characteristicscan be obtained. Since the on-state current can be increased, it ispossible to reduce the size of the transistor 270. In addition, sincethe transistor 270 is surrounded by the conductive film 204 functioningas a first gate electrode and the conductive film 220 b functioning as asecond gate electrode, the mechanical strength of the transistor 270 canbe increased.

<2-5. Structural Example of Semiconductor Device>

Structure examples different from that of the transistor 270 in FIGS.17A to 17C are described with reference to FIGS. 18A to 18D.

FIGS. 18A and 18B illustrate a cross-sectional view illustrating avariation of the transistor 270 in FIGS. 17B and 17C. FIGS. 18C and 18Dillustrate a cross-sectional view illustrating another variation of thetransistor 270 in FIGS. 17B and 17C.

A transistor 270A in FIGS. 18A and 18B has the same structure as thetransistor 270 in FIGS. 17B and 17C except that the oxide semiconductorfilm 208 has a three-layer structure. Specifically, the oxidesemiconductor film 208 of the transistor 270A includes an oxidesemiconductor film 208 a, an oxide semiconductor film 208 b, and anoxide semiconductor film 208 c.

A transistor 270B in FIGS. 18C and 18D has the same structure as thetransistor 270 in FIGS. 17B and 17C except that the oxide semiconductorfilm 208 has a two-layer structure. Specifically, the oxidesemiconductor film 208 of the transistor 270B includes the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c.

Here, a band structure including the oxide semiconductor films 208 a,208 b, and 208 c and insulating films in contact with the oxidesemiconductor films 208 b and 208 c is described with reference to FIGS.19A and 19B.

FIG. 19A shows an example of a band structure in the thickness directionof a stacked-layer structure including the insulating film 207, theoxide semiconductor films 208 a, 208 b, and 208 c, and the insulatingfilm 214. FIG. 19B shows an example of a band structure in the thicknessdirection of a stacked-layer structure including the insulating film207, the oxide semiconductor films 208 b and 208 c, and the insulatingfilm 214. For easy understanding, energy level of the conduction bandminimum (Ec) of each of the insulating film 207, the oxide semiconductorfilms 208 a, 208 b, and 208 c, and the insulating film 214 is shown inthe band structures.

In the band structure of FIG. 19A, a silicon oxide film is used as eachof the insulating film 207 and the insulating film 214, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=1:1:1.2, is used as the oxidesemiconductor film 208 a, an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=4:2:4.1, is used as the oxide semiconductor film 208 b, and anoxide semiconductor film formed using a metal oxide target having anatomic ratio of metal elements, In:Ga:Zn=1:1:1.2, is used as the oxidesemiconductor film 208 c.

In the band structure of FIG. 19B, a silicon oxide film is used as eachof the insulating film 207 and the insulating film 214, an oxidesemiconductor film formed using a metal oxide target having an atomicratio of metal elements, In:Ga:Zn=4:2:4.1, is used as the oxidesemiconductor film 208 b, and an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:1:1.2, is used as the oxide semiconductor film 208 c.

As illustrated in FIGS. 19A and 19B, the energy level of the conductionband minimum gradually changes between the oxide semiconductor film 208a and the oxide semiconductor film 208 b and between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c. Inother words, the energy level of the conduction band minimum iscontinuously changed or continuously connected. To obtain such a bandstructure, there exists no impurity, which forms a defect state such asa trap center or a recombination center, at the interface between theoxide semiconductor film 208 a and the oxide semiconductor film 208 b orat the interface between the oxide semiconductor film 208 b and theoxide semiconductor film 208 c.

To form a continuous junction between the oxide semiconductor film 208 aand the oxide semiconductor film 208 b and between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c, it isnecessary to form the films successively without exposure to the air byusing a multi-chamber deposition apparatus (sputtering apparatus)provided with a load lock chamber.

With the band structure of FIG. 19A or FIG. 19B, the oxide semiconductorfilm 208 b serves as a well, and a channel region is formed in the oxidesemiconductor film 208 b in the transistor with the stacked-layerstructure.

By providing the oxide semiconductor film 208 a and the oxidesemiconductor film 208 c, the oxide semiconductor film 208 b can bedistanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the energy level of the conduction band minimum (Ec) of the oxidesemiconductor film 208 b functioning as a channel region, so thatelectrons are likely to be accumulated in the trap states. When theelectrons are accumulated in the trap states, the electrons becomenegative fixed electric charge, so that the threshold voltage of thetransistor is shifted in the positive direction. Therefore, it ispreferable that the trap states be closer to the vacuum level than theenergy level of the conduction band minimum (Ec) of the oxidesemiconductor film 208 b. Such a structure inhibits accumulation ofelectrons in the trap states. As a result, the on-state current and thefield-effect mobility of the transistor can be increased.

The energy level of the conduction band minimum of each of the oxidesemiconductor films 208 a and 208 c is closer to the vacuum level thanthat of the oxide semiconductor film 208 b. Typically, a difference inenergy level between the conduction band minimum of the oxidesemiconductor film 208 b and the conduction band minimum of each of theoxide semiconductor films 208 a and 208 c is 0.15 eV or more or 0.5 eVor more and 2 eV or less or 1 eV or less. That is, the differencebetween the electron affinity of each of the oxide semiconductor films208 a and 208 c and the electron affinity of the oxide semiconductorfilm 208 b is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eVor less.

In such a structure, the oxide semiconductor film 208 b serves as a mainpath of current and functions as a channel region. In addition, sincethe oxide semiconductor films 208 a and 208 c each include one or moremetal elements included in the oxide semiconductor film 208 b in which achannel region is formed, interface scattering is less likely to occurat the interface between the oxide semiconductor film 208 a and theoxide semiconductor film 208 b or at the interface between the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c. Thus,the transistor can have high field-effect mobility because the movementof carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 208 a and 208 c fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 208 a and 208c. Alternatively, a material which has a smaller electron affinity (adifference in energy level between the vacuum level and the conductionband minimum) than the oxide semiconductor film 208 b and has adifference in energy level in the conduction band minimum from the oxidesemiconductor film 208 b (band offset) is used for the oxidesemiconductor films 208 a and 208 c. Furthermore, to inhibit generationof a difference in threshold voltage due to the value of the drainvoltage, it is preferable to form the oxide semiconductor films 208 aand 208 c using a material whose energy level of the conduction bandminimum is closer to the vacuum level than that of the oxidesemiconductor film 208 b by 0.2 eV or more, preferably 0.5 eV or more.

It is preferable that the oxide semiconductor films 208 a and 208 c nothave a spinel crystal structure. This is because if the oxidesemiconductor films 208 a and 208 c have a spinel crystal structure,constituent elements of the conductive films 212 a and 212 b might bediffused to the oxide semiconductor film 208 b at the interface betweenthe spinel crystal structure and another region. Note that each of theoxide semiconductor film 208 a and 208 c is preferably a CAAC-OS, inwhich case a higher blocking property against constituent elements ofthe conductive films 212 a and 212 b, for example, copper elements, isobtained.

The thickness of each of the oxide semiconductor films 208 a and 208 cis greater than or equal to a thickness that is capable of inhibitingdiffusion of the constituent elements of the conductive films 212 a and212 b to the oxide semiconductor film 208 b, and less than a thicknessthat inhibits supply of oxygen from the insulating film 214 to the oxidesemiconductor film 208 b. For example, when the thickness of each of theoxide semiconductor films 208 a and 208 c is greater than or equal to 10nm, diffusion of the constituent elements of the conductive films 212 aand 212 b to the oxide semiconductor film 208 b can be inhibited. Whenthe thickness of each of the oxide semiconductor films 208 a and 208 cis less than or equal to 100 nm, oxygen can be effectively supplied fromthe insulating film 214 to the oxide semiconductor film 208 b.

Although the example where an oxide semiconductor film formed using ametal oxide target having an atomic ratio of metal elements,In:Ga:Zn=1:1:1.2, is used as each of the oxide semiconductor films 208 aand 208 c is described in this embodiment, one embodiment of the presentinvention is not limited thereto. For example, an oxide semiconductorfilm formed using a metal oxide target having an atomic ratio of metalelements, In:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, orIn:Ga:Zn=1:3:6, may be used as each of the oxide semiconductor films 208a and 208 c.

When the oxide semiconductor films 208 a and 208 c are formed using ametal oxide target having an atomic ratio of In:Ga:Zn=1:1:1, the oxidesemiconductor films 208 a and 208 c have an atomic ratio ofIn:Ga:Zn=1:β1 (0<β1≤2)z:β2 (0<β2≤3) in some cases. When the oxidesemiconductor films 208 a and 208 c are formed using a metal oxidetarget having an atomic ratio of In:Ga:Zn=1:3:4, the oxide semiconductorfilms 208 a and 208 c have an atomic ratio of In:Ga:Zn=1:β3 (1≤β3≤5):β4(2≤β4≤6) in some cases. When the oxide semiconductor films 208 a and 208c are formed using a metal oxide target having an atomic ratio ofIn:Ga:Zn=1:3:6, the oxide semiconductor films 208 a and 208 c have anatomic ratio of In:Ga:Zn=1:β5 (1≤β5≤5):β6 (4≤β6≤8) in some cases.

The drawings illustrate an example where the oxide semiconductor film208 in the transistors 200 and 270 and the oxide semiconductor film 208c in the transistors 270A and 270B have a small thickness in a regionwhich is not covered with the conductive films 212 a and 212 b, that is,an example where part of the oxide semiconductor film has a depressedportion. However, one embodiment of the present invention is not limitedthereto, and the oxide semiconductor film does not necessarily have adepressed portion in a region which is not covered with the conductivefilms 212 a and 212 b. FIGS. 20A to 20D illustrate examples in thiscase. FIGS. 20A to 20D are cross-sectional views illustrating examplesof the semiconductor device. FIGS. 20A and 20B illustrate a structurewhere the oxide semiconductor film 208 in the transistor 200 does nothave a depressed portion, and FIGS. 20C and 20D illustrate a structurewhere the oxide semiconductor film 208 in the transistor 270B does nothave a depressed portion.

The structures of the transistors of this embodiment can be freelycombined with each other.

<2-6. Manufacturing Method of Semiconductor Device>

Next, a method for manufacturing the semiconductor device of oneembodiment of the present invention is described with reference todrawings.

The films included in the semiconductor device of one embodiment of thepresent invention (i.e., the conductive film, the insulating film, theoxide semiconductor film, and the like) can be formed by a sputteringmethod, a chemical vapor deposition (CVD) method, a plasma-enhanced CVD(PECVD) method, a vacuum evaporation method, or a pulsed laserdeposition (PLD) method. Without limitation thereto, the films may beformed by a coating method, a printing method, a thermal CVD method, oran atomic layer deposition (ALD) method, for example. By a thermal CVDmethod such as a metal organic chemical vapor deposition (MOCVD) method,the conductive film, the insulating film, the oxide semiconductor film,and the like may be formed.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a timewhile the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and the source gas and the oxidizer react with eachother in the vicinity of the substrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced when orafter the first source gas is introduced so that the source gases arenot mixed, and then a second source gas is introduced. Note that in thecase where the first source gas and the inert gas are introduced at atime, the inert gas serves as a carrier gas, and the inert gas may alsobe introduced at the same time as the second source gas. Alternatively,the first source gas may be exhausted by vacuum evacuation instead ofthe introduction of the inert gas, and then the second source gas may beintroduced. The first source gas is adsorbed on the surface of thesubstrate to form a first layer; then the second source gas isintroduced to react with the first layer; as a result, a second layer isstacked over the first layer, so that a thin film is formed. Thesequence of the gas introduction is repeated plural times until adesired thickness is obtained, whereby a thin film with excellent stepcoverage can be formed. The thickness of the thin film can be adjustedby the number of repetition times of the sequence of the gasintroduction; thus, an ALD method makes it possible to accurately adjustthe film thickness and thus is suitable for manufacturing a minute FET.

The above conductive films, insulating films, oxide semiconductor films,the metal oxide films, and the like can be formed by a thermal CVDmethod such as an MOCVD method. To form an In—Ga—Zn—O film, for example,trimethylindium, trimethylgallium, and dimethylzinc can be used. Notethat the chemical formula of trimethylindium is In(CH₃)₃. The chemicalformula of trimethylgallium is Ga(CH₃)₃. The chemical formula ofdimethylzinc is Zn(CH₃)₂. Without limitation to the above combination,triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead oftrimethylgallium, and diethylzinc (chemical formula: Zn(C₂H₅)₂) can beused instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed with adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and a hafnium precursor compound (hafnium alkoxideor hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) areused. Note that the chemical formula of tetrakis(dimethylamide)hafniumis Hf[N(CH₃)₂]₄. Examples of another material liquid includetetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed with adeposition apparatus employing ALD, two kinds of gases, i.e., H₂O as anoxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed with adeposition apparatus employing ALD, hexachlorodisilane is adsorbed onthe surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed with adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed with a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas) are sequentially introduced a plurality oftimes to form an In—O layer, then a Ga(CH₃)₃ gas and an O₃ gas) are usedto form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas) are used toform a Zn—O layer. Note that the order of these layers is not limited tothis example. A mixed compound layer such as an In—Ga—O layer, anIn—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases.Note that although an H₂O gas which is obtained by bubbling with aninert gas such as Ar may be used instead of an O₃ gas), it is preferableto use an O₃ gas), which does not contain H. Instead of an In(CH₃)₃ gas,an In(C₂H₅)₃ may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas maybe used. Furthermore, Zn(CH₃)₂ gas may be used.

[Method 1 for Manufacturing Semiconductor Device]

First, a method for manufacturing the transistor 270B that is asemiconductor device of one embodiment of the present invention, whichis illustrated in FIGS. 18C and 18D, is described with reference toFIGS. 21A to 21F, FIGS. 22A to 22F, and FIGS. 23A to 23F. FIGS. 21A to21F, FIGS. 22A to 22F, and FIGS. 23A to 23F are cross-sectional viewsillustrating a method for manufacturing a semiconductor device. FIGS.21A, 21C, and 21E, FIGS. 22A, 22C, and 22E, and FIGS. 23A, 23C, and 23Eare cross-sectional views in the channel length direction, and FIGS.21B, 21D, and 21F, FIGS. 22B, 22D, and 22F, and FIGS. 23B, 23D, and 23Fare cross-sectional views in the channel width direction.

First, a conductive film is formed over the substrate 202 and processedthrough a lithography process and an etching process, whereby theconductive film 204 functioning as a gate electrode is formed. Then, theinsulating films 206 and 207 functioning as gate insulating films areformed over the conductive film 204 (see FIGS. 21A and 21B).

In this embodiment, a glass substrate is used as the substrate 202, andas the conductive film 204 functioning as a gate electrode, a100-nm-thick tungsten film is formed by a sputtering method. A400-nm-thick silicon nitride film as the insulating film 206 and a50-nm-thick silicon oxynitride film as the insulating film 207 areformed by a PECVD method.

The insulating film 206 can have a stacked-layer structure of siliconnitride films. Specifically, the insulating film 206 can have athree-layer stacked-layer structure of a first silicon nitride film, asecond silicon nitride film, and a third silicon nitride film. Anexample of the three-layer stacked-layer structure can be formed asfollows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the conditions where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus, the pressure in the reaction chamber is controlledto 100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the conditions where silane at a flow rate of 200 sccm,nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rateof 2000 sccm are supplied as a source gas to the reaction chamber of thePECVD apparatus, the pressure in the reaction chamber is controlled to100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50nm under the conditions where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus, the pressure in the reactionchamber is controlled to 100 Pa, and a power of 2000 W is supplied usinga 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can be each formed at asubstrate temperature of 350° C. or lower.

When the insulating film 206 has the three-layer stacked-layer structureof silicon nitride films, for example, in the case where a conductivefilm containing copper (Cu) is used as the conductive film 204, thefollowing effect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu)element from the conductive film 204. The second silicon nitride filmhas a function of releasing hydrogen and can improve withstand voltageof the insulating film serving as a gate insulating film. The thirdsilicon nitride film releases a small amount of hydrogen and can inhibitdiffusion of hydrogen released from the second silicon nitride film.

The insulating film 207 is preferably an insulating film containingoxygen to improve characteristics of an interface with the oxidesemiconductor film 208 (specifically the oxide semiconductor film 208 b)formed later.

Next, a stacked-layer film of oxide semiconductor films is formed overthe insulating film 207 and is processed into a desired shape, so thatthe island-shaped oxide semiconductor film 208 including the oxidesemiconductor film 208 b and the oxide semiconductor film 208 c isformed (see FIGS. 21C and 21D).

The oxide semiconductor film 208 is formed at a temperature higher thanor equal to room temperature and lower than 340° C., preferably higherthan or equal to room temperature and lower than or equal to 300° C.,further preferably higher than or equal to 100° C. and lower than orequal to 250° C., still further preferably higher than or equal to 100°C. and lower than or equal to 200° C. The oxide semiconductor film 208is formed while being heated, so that the crystallinity of the oxidesemiconductor film 208 can be increased. On the other hand, in the casewhere a large-sized glass substrate (e.g., the 6th generation to the10th generation) is used as the substrate 202 and the oxidesemiconductor film 208 is formed at a temperature higher than or equalto 150° C. and lower than 340° C., the substrate 202 might be changed inshape (distorted or warped). In the case where a large-sized glasssubstrate is used, the change in the shape of the glass substrate can besuppressed by forming the oxide semiconductor film 208 at a temperaturehigher than or equal to 100° C. and lower than 150° C.

The oxide semiconductor films 208 b and 208 c may be formed at the samesubstrate temperature or different substrate temperatures. Note that theoxide semiconductor films 208 b and 208 c are preferably formed at thesame substrate temperature, in which case the manufacturing cost canreduced.

In this embodiment, an oxide semiconductor film to be the oxidesemiconductor film 208 b is deposited by a sputtering method using anIn—Ga—Zn metal oxide target (having an atomic ratio ofIn:Ga:Zn=4:2:4.1), and an oxide semiconductor film to be the oxidesemiconductor film 208 c is successively deposited in a vacuum by asputtering method using an In—Ga—Zn metal oxide target (having an atomicratio of In:Ga:Zn=1:1:1.2). The substrate temperature during thedeposition of the oxide semiconductor film to be the oxide semiconductorfilm 208 is 170° C. Oxygen and argon are used as deposition gases forthe oxide semiconductor film to be the oxide semiconductor film 208.

In the case where the oxide semiconductor film is deposited by asputtering method, as a sputtering gas, a rare gas (typically argon),oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate.In the case of using the mixed gas of a rare gas and oxygen, theproportion of oxygen to a rare gas is preferably increased. In addition,increasing the purity of a sputtering gas is necessary. For example,when a gas which is highly purified to have a dew point of −40° C. orlower, preferably −80° C. or lower, further preferably −100° C. orlower, still further preferably −120° C. or lower, is used as asputtering gas, i.e., the oxygen gas or the argon gas, entry of moistureor the like into the oxide semiconductor film can be minimized.

In the case where the oxide semiconductor film is deposited by asputtering method, the sputtering gas containing oxygen is preferablyused. When the oxide semiconductor film is deposited using thesputtering gas containing oxygen, oxygen can be added to a film underthe oxide semiconductor film (here, the insulating film 207) at the sametime as the formation of the oxide semiconductor film. Therefore, anoxygen-excess region can be provided in the insulating film 207.

In the case where the oxide semiconductor film is formed by a sputteringmethod, a chamber in a sputtering apparatus is preferably evacuated tobe a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa)with an adsorption vacuum evacuation pump such as a cryopump in order toremove water or the like, which serves as an impurity for the oxidesemiconductor film, as much as possible. Alternatively, a turbomolecular pump and a cold trap are preferably combined so as to preventa backflow of a gas, especially a gas containing carbon or hydrogen froman exhaust system to the inside of the chamber.

Next, the conductive film 212 to be the source electrode and the drainelectrode is formed over the insulating film 207 and the oxidesemiconductor film 208 by a sputtering method (see FIGS. 21E and 21F).

In this embodiment, the conductive film 212 is formed of a 400-nm-thickaluminum film is stacked over a 50-nm-thick tungsten film by asputtering method. Although the conductive film 212 has a two-layerstructure in this embodiment, one embodiment of the present invention isnot limited thereto. For example, the conductive film 212 may have athree-layer structure in which a 400-nm-thick aluminum film is stackedover a 50-nm-thick tungsten film, and a 100-nm-thick titanium film isstacked over the 400-nm-thick aluminum film.

Next, the conductive film 212 is processed into desired shapes, so thatthe separate conductive films 212 a and 212 b are formed (see FIGS. 22Aand 22B).

In this embodiment, the conductive film 212 is processed with a dryetching apparatus. Note that the method for processing the conductivefilm 212 is not limited thereto, and a wet etching apparatus may beused, for example. When the conductive film 212 is processed, a finerpattern can be formed with a dry etching apparatus than with a wetetching apparatus. On the other hand, the conductive film 212 can beprocessed with a wet etching apparatus at lower manufacturing cost thanwith a dry etching apparatus.

After the conductive films 212 a and 212 b are formed, a surface (on theback channel side) of the oxide semiconductor film 208 (specifically,the oxide semiconductor film 208 c) may be cleaned. The cleaning may beperformed, for example, using a chemical solution such as phosphoricacid. The cleaning using a chemical solution such as a phosphoric acidcan remove impurities (e.g., an element included in the conductive films212 a and 212 b) attached to the surface of the oxide semiconductor film208 c. Note that the cleaning is not necessarily performed, and thus thecleaning may be unnecessary.

In the step of forming the conductive films 212 a and 212 b and/or thecleaning step, the thickness of a region of the oxide semiconductor film208 which is not covered by the conductive films 212 a and 212 b mightbe reduced. For example, a region where the oxide semiconductor film 208c has a smaller thickness than the oxide semiconductor film 208 b isformed in some cases.

Next, the insulating films 214 and 216 are formed over the oxidesemiconductor film 208 and the conductive films 212 a and 212 b (seeFIGS. 22C and 22D).

Note that after the insulating film 214 is formed, the insulating film216 is preferably formed in succession without exposure to the air.After the insulating film 214 is formed, the insulating film 216 isformed in succession without exposure to the air while at least one ofthe flow rate of a source gas, pressure, a high-frequency power, and asubstrate temperature is adjusted, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 214 and the insulating film 216 can bereduced and oxygen in the insulating films 214 and 216 can be moved tothe oxide semiconductor film 208; accordingly, the amount of oxygenvacancies in the oxide semiconductor film 208 can be reduced.

As the insulating film 214, a silicon oxynitride film can be formed by aPECVD method, for example. In this case, a deposition gas containingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas containing silicon includesilane, disilane, trisilane, and silane fluoride. Examples of theoxidizing gas include dinitrogen monoxide and nitrogen dioxide. Aninsulating film containing nitrogen and having a small number of defectscan be formed as the insulating film 214 by a PECVD method under theconditions where the flow rate of the oxidizing gas is higher than 20times and lower than 100 times, preferably higher than or equal to 40times and lower than or equal to 80 times, that of the deposition gas;and the pressure in a treatment chamber is lower than 100 Pa, preferablylower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as theinsulating film 214 by a PECVD method under the conditions where thesubstrate 202 is held at a temperature of 220° C., silane at a flow rateof 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are usedas a source gas, the pressure in the treatment chamber is 20 Pa, and ahigh-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the powerdensity) is supplied to parallel-plate electrodes.

As the insulating film 216, a silicon oxide film or a silicon oxynitridefilm is formed under the following conditions: the substrate placed in atreatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature higher than or equal to 180° C. and lower than orequal to 350° C.; the pressure is greater than or equal to 100 Pa andless than or equal to 250 Pa, preferably greater than or equal to 100 Paand less than or equal to 200 Pa with introduction of a source gas intothe treatment chamber; and a high-frequency power of greater than orequal to 0.17 W/cm² and less than or equal to 0.5 W/cm², preferablygreater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm²is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 216, thehigh-frequency power having the above power density is supplied to areaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the insulating film 216 becomes higher than that in thestoichiometric composition. In addition, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which contains oxygen at ahigher proportion than the stoichiometric composition and from whichpart of oxygen is released by heating.

Note that the insulating film 214 functions as a protective film for theoxide semiconductor film 208 in the step of forming the insulating film216. Therefore, the insulating film 216 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 208 is reduced.

Note that in the deposition conditions of the insulating film 216, whenthe flow rate of the deposition gas containing silicon with respect tothe oxidizing gas is increased, the amount of defects in the insulatingfilm 216 can be reduced. As a typical example, it is possible to form anoxide insulating layer in which the amount of defects is small, i.e.,the spin density of a signal which appears at g=2.001 originating from adangling bond of silicon is lower than 6×10¹⁷ spins/cm³, preferablylower than or equal to 3×10¹⁷ spins/cm³, further preferably lower thanor equal to 1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, thereliability of the transistor can be improved.

Heat treatment (hereinafter referred to as first heat treatment) ispreferably performed after the insulating films 214 and 216 are formed.The first heat treatment can reduce nitrogen oxide contained in theinsulating films 214 and 216. By the first heat treatment, part ofoxygen contained in the insulating films 214 and 216 can be moved to theoxide semiconductor film 208, so that the amount of oxygen vacanciesincluded in the oxide semiconductor film 208 can be reduced.

The temperature of the first heat treatment is typically lower than 400°C., preferably lower than 375° C., further preferably higher than orequal to 150° C. and lower than or equal to 350° C. The first heattreatment may be performed under an atmosphere of nitrogen, oxygen,ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, and the like). Note that an electric furnace, arapid thermal anneal (RTA) apparatus, or the like can be used for thefirst heat treatment, in which it is preferable that hydrogen, water,and the like not be contained in the nitrogen, oxygen, ultra-dry air, ora rare gas.

Next, a barrier film 230 is formed over the insulating film 216, andoxygen 240 is added to the insulating film 216, the insulating film 214,or the oxide semiconductor film 208 through the barrier film 230 (seeFIGS. 22E and 22F).

In FIGS. 22E and 22F, oxygen added to the insulating film 214 or theinsulating film 216 is schematically shown by arrows of broken lines.

The barrier film 230 allows oxygen to pass through and inhibits releaseof oxygen. The barrier film 230 includes, for example, oxygen and metal(at least one element selected from indium, zinc, titanium, aluminum,tungsten, tantalum, molybdenum, hafnium, and yttrium). In particular,the barrier film 230 preferably includes indium tin oxide (also referredto as ITO), indium tin silicon oxide (In—Sn—Si oxide; hereinafter alsoreferred to as ITSO), or indium oxide because an uneven surface can befavorably covered with such a material. Alternatively, as the barrierfilm 230, the above-described oxide semiconductor film (having an atomicratio of In:Ga:Zn=1:1:1, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:6,or In:Ga:Zn=4:2:3, for example) may be used.

The barrier film 230 can be formed by a sputtering method. When thebarrier film 230 is thin, oxygen release from the insulating film 216 tothe outside is difficult to suppress in some cases. On the other hand,when the barrier film 230 is thick, oxygen cannot be favorably added tothe insulating film 216 in some cases. Accordingly, the thickness of thebarrier film 230 is preferably greater than or equal to 1 nm and lessthan or equal to 20 nm, or greater than or equal to 2 nm and less thanor equal to 10 nm. In this embodiment, the barrier film 230 is a5-nm-thick ITSO film.

As examples of the method for adding the oxygen 240 to the insulatingfilm 216 through the barrier film 230, there are an ion doping method,an ion implantation method, and a plasma treatment method. Depending onthe apparatus or conditions for adding the oxygen 240, the oxygen 240can be added to the insulating film 214 or the oxide semiconductor film208 under the insulating film 216 in some cases. As the oxygen 240,excess oxygen, an oxygen radical, or the like can be used. The oxygen240 can be effectively added to the insulating film 216 while a bias isapplied to the substrate side. As the bias, for example, an ashingapparatus is used, and power density of the bias applied between a pairof electrodes included in the ashing apparatus can be greater than orequal to 1 W/cm² and less than or equal to 5 W/cm². By providing thebarrier film 230 over the insulating film 216 and adding the oxygen 240,the barrier film 230 functions as a protective film for inhibitingrelease of oxygen from the insulating film 216. Thus, a larger amount ofoxygen can be added to the insulating film 216.

After the oxygen 240 is added to the insulating film 216 through thebarrier film 230, heat treatment (hereinafter referred to as second heattreatment) may be performed. The second heat treatment can be performedunder conditions similar to those of the first heat treatment.

Next, the barrier film 230 is removed to expose a surface of theinsulating film 216, and then, the insulating film 218 is formed overthe insulating film 216 (see FIGS. 23A and 23B).

When the barrier film 230 is removed, part of the insulating film 216 isalso removed in some cases. A method for removing the barrier film 230is, for example, a dry etching method, a wet etching method, or acombination of a dry etching method and a wet etching method. In thisembodiment, a wet etching method is used to remove the barrier film 230.A wet etching method is preferably used as the method for removing thebarrier film 230 because of low manufacturing cost.

The insulating film 218 can be formed by a sputtering method or a PECVDmethod, for example. In the case where the insulating film 218 is formedby a PECVD method, for example, the substrate temperature is lower than400° C., preferably lower than 375° C., further preferably higher thanor equal to 180° C. and lower than or equal to 350° C. The substratetemperature at which the insulating film 218 is formed is preferablywithin the above range because a dense film can be formed. Furthermore,when the substrate temperature at which the insulating film 218 isformed is within the above range, oxygen or excess oxygen in theinsulating films 214 and 216 can be moved to the oxide semiconductorfilm 208.

After the insulating film 218 is formed, heat treatment similar to thesecond heat treatment (hereinafter referred to as third heat treatment)may be performed. Through such heat treatment at lower than 400° C.,preferably lower than 375° C., further preferably higher than or equalto 180° C. and lower than or equal to 350° C. after the addition of theoxygen 240 to the insulating film 216, oxygen or excess oxygen in theinsulating film 216 can be moved to the oxide semiconductor film 208(particularly, the oxide semiconductor film 208 b) and compensate oxygenvacancies in the oxide semiconductor film 208.

Oxygen moved to the oxide semiconductor film 208 is described withreference to FIGS. 24A and 24B. FIGS. 24A and 24B are model diagramsillustrating oxygen moved to the oxide semiconductor film 208 due to thesubstrate temperature at the time of forming the insulating film 218(typically, lower than 375° C.) or the third heat treatment after theformation of the insulating film 218 (typically, lower than 375° C.). InFIGS. 24A and 24B, oxygen (oxygen radicals, oxygen atoms, or oxygenmolecules) moved to the oxide semiconductor film 208 is shown by arrowsof broken lines.

In the oxide semiconductor film 208 in FIGS. 24A and 24B, oxygenvacancies are compensated with oxygen moved from films in contact withthe oxide semiconductor film 208 (here, the insulating film 207 and theinsulating film 214). Specifically, in the semiconductor device of oneembodiment of the present invention, the insulating film 207 includes anoxygen-excess region because an oxygen gas is used at the time offorming the oxide semiconductor film 208 by sputtering and oxygen isadded to the insulating film 207. Since oxygen is added through thebarrier film 230, the insulating films 214 and 216 also include anoxygen-excess region. In the oxide semiconductor film 208 between theinsulating films including the oxygen-excess regions, oxygen vacanciescan be favorably compensated.

Furthermore, the insulating film 206 is provided under the insulatingfilm 207, and the insulating film 218 is provided over the insulatingfilms 214 and 216. When the insulating films 206 and 218 are formedusing a material having low oxygen permeability, e.g., silicon nitride,oxygen contained in the insulating films 207, 214, and 216 can beconfined to the oxide semiconductor film 208 side; thus, oxygen can befavorably moved to the oxide semiconductor film 208.

In the case where a silicon nitride film is formed by a PECVD method asthe insulating film 218, a deposition gas containing silicon, nitrogen,and ammonia are preferably used as a source gas. As the source gas, asmall amount of ammonia compared to the amount of nitrogen is used,whereby ammonia is dissociated in the plasma and activated species aregenerated. The activated species cleave a bond between silicon andhydrogen which are contained in a deposition gas containing silicon anda triple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which bonds between silicon andnitrogen are promoted and bonds between silicon and hydrogen are few,can be formed. On the other hand, when the amount of ammonia withrespect to nitrogen is large, decomposition of a deposition gascontaining silicon and decomposition of nitrogen are not promoted, sothat a sparse silicon nitride film in which bonds between silicon andhydrogen remain and defects are increased is formed. Therefore, in asource gas, the flow rate of the nitrogen is set to be preferably 5times or more and 50 times or less, more preferably 10 times or more and50 times or less the flow rate of the ammonia.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thicksilicon nitride film is formed as the insulating film 218 using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and high-frequency power of 1000 Wis supplied to parallel-plate electrodes with a 27.12 MHz high-frequencypower source. Note that the PECVD apparatus is a parallel-plate PECVDapparatus in which the electrode area is 6000 cm², and the power perunit area (power density) into which the supplied power is converted is1.7×10⁻¹ W/cm².

Next, a mask is formed over the insulating film 218 through alithography process, and the opening 252 c is formed in a desired regionin the insulating films 214, 216, and 218. In addition, a mask is formedover the insulating film 218 through a lithography process, and theopenings 252 a and 252 b are formed in desired regions in the insulatingfilms 206, 207, 214, 216, and 218. Note that the opening 252 c reachesthe conductive film 212 b. The openings 252 a and 252 b reach theconductive film 204 (see FIGS. 23C and 23D).

Note that the openings 252 a and 252 b and the opening 252 c may beformed in the same step or may be formed by different steps. In the casewhere the openings 252 a and 252 b and the opening 252 c are formed inthe same step, for example, a gray-tone mask or a half-tone mask can beused. Moreover, the openings 252 a and 252 b may be formed in somesteps. For example, openings are formed in the insulating films 206 and207 in advance, and then, openings are formed in the insulating films214, 216, and 218 over the openings.

Next, a conductive film is formed over the insulating film 218 to coverthe openings 252 a, 252 b, and 252 c and processed into desired shapes,so that the conductive films 220 a and 220 b are formed (see FIGS. 23Eand 23F).

For the conductive film to be the conductive films 220 a and 220 b, forexample, a material including one of indium (In), zinc (Zn), and tin(Sn) can be used. In particular, for the conductive films 220 a and 220b, a light-transmitting conductive material such as indium oxideincluding tungsten oxide, indium zinc oxide including tungsten oxide,indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (ITO), indium zinc oxide, or indium tinsilicon oxide (ITSO) can be used. Moreover, the conductive film to bethe conductive films 220 a and 220 b can be formed by a sputteringmethod, for example. In this embodiment, a 110-nm-thick ITSO film isformed by a sputtering method.

Through the above process, the transistor 270B illustrated in FIGS. 18Cand 18D can be manufactured.

In the entire manufacturing process of the transistor 270B, thesubstrate temperature is preferably lower than 400° C., furtherpreferably lower than 375° C., still further preferably higher than orequal to 180° C. and lower than or equal to 350° C. because the changein shape of the substrate (distortion or warp) can be reduced even whena large-sized substrate is used. In the manufacturing process of thetransistor 270B, the substrate is heated at the following temperatures:the temperature in the formation of the insulating films 206 and 207(lower than 400° C., preferably higher than or equal to 250° C. andlower than or equal to 350° C.), the temperature in the formation of theoxide semiconductor film 208 (higher than or equal to room temperatureand lower than 340° C., preferably higher than or equal to 100° C. andlower than or equal to 200° C., further preferably higher than or equalto 100° C. and lower than 150° C.), the temperature in the formation ofthe insulating films 216 and 218 (lower than 400° C., preferably lowerthan 375° C., further preferably higher than or equal to 180° C. andlower than or equal to 350° C.), the temperature in the first heattreatment or the second heat treatment after the addition of the oxygen240 (lower than 400° C., preferably lower than 375° C., furtherpreferably higher than or equal to 180° C. and lower than or equal to350° C.), and the like.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

[Method 2 for Manufacturing Semiconductor Device]

A manufacturing method different from [Method 1 for manufacturingsemiconductor device] is described below.

First, steps up to the step illustrated in FIGS. 22C and 22D areperformed in a similar manner to that of [Method 1 for manufacturingsemiconductor device]. Next, the barrier film 230 is formed asillustrated in FIGS. 22E and 22F, and the oxygen 240 is not added. Then,the step illustrated in FIGS. 23A and 23B is not performed, and thesteps illustrated in FIGS. 23C and 23D and FIGS. 23E and 23F areperformed.

In this case, for the barrier film 230, a material having a highinsulating property is selected from the above materials. For thebarrier film 230 used in this manufacturing method, aluminum oxide,hafnium oxide, or yttrium oxide is preferably used.

When the barrier film 230 is formed by a sputtering method usingaluminum oxide, hafnium oxide, or yttrium oxide, the sputtering gaspreferably contains at least oxygen. Oxygen used for the sputtering gasin the formation of the barrier film 230 becomes oxygen radicals inplasma, and the oxygen and/or the oxygen radicals are added to theinsulating film 216 in some cases. Therefore, the step of adding theoxygen 240 illustrated in FIGS. 22E and 22F is not necessarilyperformed. In other words, oxygen adding treatment and the formation ofthe barrier film 230 can be performed at the same time. While thebarrier film 230 has a function of adding oxygen in the formation of thebarrier film 230 (particularly in an early stage of the formation), thebarrier film 230 has a function of blocking oxygen after the formationof the barrier film 230 (particularly in a later stage of theformation).

In the case where the barrier film 230 is formed by a sputtering methodusing aluminum oxide, for example, a mixed layer might be formed in thevicinity of the interface between the insulating film 216 and thebarrier film 230. For example, when the insulating film 216 is a siliconoxynitride film, an Al_(x)Si_(y)O_(z) layer might be formed as the mixedlayer. The mixed layer may include an oxygen-excess region.

When the barrier film 230 is formed using aluminum oxide, hafnium oxide,or yttrium oxide, which have a high insulating property and a highoxygen barrier property, the step of forming the insulating film 218illustrated in FIGS. 23A and 23B is not necessarily performed. It may bepossible not to remove the barrier film 230 and use it instead of theinsulating film 218.

When the substrate temperature in the formation of the barrier film 230is lower than 400° C., preferably lower than 375° C., further preferablyhigher than or equal to 180° C. and lower than or equal to 350° C.,oxygen or excess oxygen added to the insulating film 216 can be moved tothe oxide semiconductor film 208.

By using aluminum oxide, hafnium oxide, or yttrium oxide for the barrierfilm 230 as described above, the number of manufacturing steps of thesemiconductor device can be reduced, which leads to low manufacturingcost.

[Method 3 for Manufacturing Semiconductor Device]

Next, a method for manufacturing the transistor 250 that is asemiconductor device of one embodiment of the present invention, whichis illustrated in FIGS. 15A to 15C, is described with reference to FIGS.25A to 25F and FIGS. 26A to 26F. FIGS. 25A to 25F and FIGS. 26A to 26Fare cross-sectional views illustrating a method for manufacturing asemiconductor device. FIGS. 25A, 25C, and 25E and FIGS. 26A, 26C, and26E are cross-sectional views in the channel length direction, and FIGS.25B, 25D, and 25F and FIGS. 26B, 26D, and 26F are cross-sectional viewsin the channel width direction.

First, the conductive film 204, the insulating films 206 and 207, theoxide semiconductor film 208, the insulating films 214 and 216, and thebarrier film 230 are formed over the substrate 202 (see FIGS. 25A and25B).

The description of [Method 1 for manufacturing semiconductor device] canbe referred to for the conductive film 204, the insulating films 206 and207, the oxide semiconductor film 208, the insulating films 214 and 216,and the barrier film 230.

Next, the oxygen 240 is added to the insulating film 214 through thebarrier film 230 (see FIGS. 25C and 25D).

Next, the barrier film 230 is removed. Then, a mask is formed over theinsulating film 214 through a lithography process, and the openings 251a and 251 b are formed in desired regions in the insulating films 214and 216. Note that the openings 251 a and 251 b reach the oxidesemiconductor film 208 (see FIGS. 25E and 25F).

Next, the conductive film 212 is formed over the insulating film 214 tocover the openings 251 a and 251 b (see FIGS. 26A and 26B).

Next, a mask is formed over the conductive film 212 through alithography process, and the conductive film is processed into desiredshapes, so that the conductive films 212 a and 212 b are formed (seeFIGS. 26C and 26D).

Next, the insulating film 218 is formed over the insulating film 214 andthe conductive films 212 a and 212 b (see FIGS. 26E and 26F).

Through the above process, the transistor 250 illustrated in FIGS. 15Ato 15C can be manufactured.

Note that the transistor 260 in FIGS. 16A to 16C can be manufactured insuch a manner that the insulating films 214 and 216 are left only over achannel region of the oxide semiconductor film 208 at the step offorming the openings 251 a and 251 b.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments and the examples.

Embodiment 3

In this embodiment, a sputtering apparatus and a deposition apparatus bywhich the oxide semiconductor film of one embodiment of the presentinvention can be formed are described with reference to FIGS. 27A and27B, FIGS. 28A and 28B, FIGS. 29A to 29C, FIG. 30 , FIG. 31 , FIGS. 32Aand 32B, FIG. 33 , and FIGS. 34A to 34C. In the sputtering apparatusdescribed below, a substrate, a target, and the like are provided toillustrate operation in the film formation. Note that since a substrate,a target, and the like are objects that a practitioner places asappropriate, the sputtering apparatus does not include a substrate and atarget in some cases.

<3-1. Sputtering Apparatus>

Examples of a sputtering apparatus include a parallel-plate-typesputtering apparatus and a facing-targets sputtering apparatus.Deposition using the parallel-plate-type sputtering apparatus can alsobe referred to as parallel electrode sputtering (PESP). Deposition usingthe facing-targets sputtering apparatus can also be referred to as vapordeposition sputtering (VDSP).

[Parallel-Plate-Type Sputtering Apparatus (PESP)]

First, the parallel-plate-type sputtering apparatus is described. FIG.27A is a cross-sectional view of a deposition chamber 301 of aparallel-plate-type sputtering apparatus. The deposition chamber 301 inFIG. 27A includes a target holder 320, a backing plate 310, a target300, a magnet unit 330, and a substrate holder 370. Note that the target300 is placed over the backing plate 310. The backing plate 310 isplaced over the target holder 320. The magnet unit 330 is placed underthe target 300 with the backing plate 310 positioned therebetween. Thesubstrate holder 370 faces the target 300. Note that in thisspecification, a magnet unit means a group of magnets. The magnet unitcan be replaced with “cathode”, “cathode magnet”, “magnetic member”,“magnetic part”, or the like. The magnet unit 330 includes a magnet330N, a magnet 330S, and a magnet holder 332. Note that in the magnetunit 330, the magnet 330N and the magnet 330S are placed over the magnetholder 332. The magnet 330N and the magnet 330S are spaced. When asubstrate 360 is transferred into the deposition chamber 301, thesubstrate 360 is placed in contact with the substrate holder 370.

The target holder 320 and the backing plate 310 are fixed to each otherwith a bolt and have the same potential. The target holder 320 has afunction of supporting the target 300 with the backing plate 310positioned therebetween.

The target 300 is fixed to the backing plate 310. For example, thetarget 300 can be fixed to the backing plate 310 with a bonding membercontaining a low-melting-point metal such as indium.

FIG. 27A illustrates a magnetic line of force 380 a and a magnetic lineof force 380 b formed by the magnet unit 330.

The magnetic line of force 380 a is one of magnetic lines of force thatform a horizontal magnetic field in the vicinity of the target 300. Thevicinity of the target 300 corresponds to a region in which the verticaldistance from the top surface of the target 300 is, for example, greaterthan or equal to 0 mm and less than or equal to 10 mm, in particular,greater than or equal to 0 mm and less than or equal to 5 mm.

The magnetic line of force 380 b is one of magnetic lines of force thatform a horizontal magnetic field in a plane apart from the top surfaceof the magnet unit 330 by a vertical distance d. The vertical distance dis, for example, greater than or equal to 0 mm and less than or equal to20 mm or greater than or equal to 5 mm and less than or equal to 15 mm.

Here, with the use of the strong magnet 330N and the strong magnet 330S,an intense magnetic field can be generated in the vicinity of thesubstrate 360. Specifically, the magnetic flux density of the horizontalmagnetic field in the vicinity of the substrate 360 can be greater thanor equal to 10 G and less than or equal to 100 G, preferably greaterthan or equal to 15 G and less than or equal to 60 G, further preferablygreater than or equal to 20 G and less than or equal to 40 G.

Note that the magnetic flux density of the horizontal magnetic field maybe measured when the magnetic flux density of the vertical magneticfield is 0 G.

By setting the magnetic flux density of the magnetic field in thedeposition chamber 301 to be in the above range, an oxide semiconductorfilm with high density and high crystallinity can be deposited. Thedeposited oxide semiconductor film hardly includes plural kinds ofcrystal phases and has a substantially-single crystalline phase.

FIG. 27B is a top view of the magnet unit 330. In the magnet unit 330,the circular or substantially circular magnet 330N and the circular orsubstantially circular magnet 330S are fixed to the magnet holder 332.The magnet unit 330 can be rotated about a normal vector at the centerof the top surface of the magnet unit 330 or a normal vectorsubstantially at the center of the top surface of the magnet unit 330.For example, the magnet unit 330 may be rotated with a beat (alsoreferred to as rhythm, pulse, frequency, period, cycle, or the like) ofgreater than or equal to 0.1 Hz and less than or equal to 1 kHz.

Thus, a region where a magnetic field on the target 300 is intensechanges as the magnet unit 330 is rotated. The region with an intensemagnetic field is a high-density plasma region; thus, sputtering of thetarget 300 easily occurs in the vicinity of the region. For example,when the region with an intense magnetic field is fixed, only a specificregion of the target 300 is used. In contrast, when the magnet unit 330is rotated as shown in FIG. 27B, the target 300 can be uniformly used.By rotating the magnet unit 330, a film with a uniform thickness and afilm with uniform quality can be deposited.

By rotating the magnet unit 330, the direction of the magnetic line offorce in the vicinity of the substrate 360 can also be changed.

Although the magnet unit 330 is rotated in this example, one embodimentof the present invention is not limited to this example. For example,the magnet unit 330 may be oscillated vertically or horizontally. Forexample, the magnet unit 330 may be oscillated with a beat of greaterthan or equal to 0.1 Hz and less than or equal to 1 kHz. Alternatively,the target 300 may be rotated or moved. For example, the target 300 maybe rotated or moved with a beat of greater than or equal to 0.1 Hz andless than or equal to 1 kHz. Further alternatively, the direction of amagnetic line of force in the vicinity of the substrate 360 may bechanged relatively by rotating the substrate 360. These methods may becombined.

The deposition chamber 301 may have a groove portion inside or under thebacking plate 310. By making fluid (air, nitrogen, a rare gas, water,oil, or the like) flow through the groove portion, discharge anomaly dueto an increase in the temperature of the target 300 or damage to thedeposition chamber 301 due to deformation of a component can beprevented in the sputtering. In that case, the backing plate 310 and thetarget 300 are preferably adhered to each other with a bonding memberbecause the cooling capability is increased.

A gasket is preferably provided between the target holder 320 and thebacking plate 310, in which case an impurity is less likely to enter thedeposition chamber 301 from the outside or the groove portion.

In the magnet unit 330, the magnet 330N and the magnet 330S are placedsuch that their surfaces on the target 300 side have oppositepolarities. Here, the case where the pole of the magnet 330N on thetarget 300 side is the north pole and the pole of the magnet 330S on thetarget 300 side is the south pole is described. Note that the layout ofthe magnets and the poles in the magnet unit 330 are not limited tothose described here or those illustrated in FIG. 27A.

In the deposition, a potential V1 applied to a terminal V1 connected tothe target holder 320 is, for example, lower than a potential V2 appliedto a terminal V2 connected to the substrate holder 370. The potential V2applied to the terminal V2 connected to the substrate holder 370 is, forexample, the ground potential. A potential V3 applied to a terminal V3connected to the magnet holder 332 is, for example, the groundpotential. Note that the potentials applied to the terminals V1, V2, andV3 are not limited to the above description. Not all the target holder320, the substrate holder 370, and the magnet holder 332 are necessarilysupplied with potentials. For example, the substrate holder 370 may beelectrically floating. Note that although the potential V1 is applied tothe terminal V1 connected to the target holder 320 (i.e., a DCsputtering method is employed) in the example illustrated in FIG. 27A,one embodiment of the present invention is not limited thereto. Forexample, it is possible to employ what is called an RF sputteringmethod, in which case a high-frequency power supply with a frequency of13.56 MHz or 27.12 MHz, for example, is connected to the target holder320.

FIG. 27A illustrates an example where the backing plate 310 and thetarget holder 320 are not electrically connected to the magnet unit 330and the magnet holder 332, but electrical connection is not limitedthereto. For example, the backing plate 310 and the target holder 320may be electrically connected to the magnet unit 330 and the magnetholder 332, and the backing plate 310, the target holder 320, the magnetunit 330, and the magnet holder 332 may have the same potential.

To increase the crystallinity of the formed oxide semiconductor film,the temperature of the substrate 360 may be set high. By setting thetemperature of the substrate 360 high, migration of sputtered particlesin the vicinity of the substrate 360 can be promoted. Thus, an oxidesemiconductor film with higher density and higher crystallinity can bedeposited. Note that the temperature of the substrate 360 is, forexample, higher than or equal to room temperature and lower than 340°C., preferably higher than or equal to room temperature and lower thanor equal to 300° C., further preferably higher than or equal to 100° C.and lower than or equal to 250° C., still further preferably higher thanor equal to 100° C. and lower than or equal to 200° C.

The vertical distance between the target 300 and the substrate 360 isgreater than or equal to 10 mm and less than or equal to 600 mm,preferably greater than or equal to 20 mm and less than or equal to 400mm, more preferably greater than or equal to 30 mm and less than orequal to 200 mm, further more preferably greater than or equal to 40 mmand less than or equal to 100 mm. Within the above range, the verticaldistance between the target 300 and the substrate 360 is small enough tosuppress a decrease in the energy of the sputtered particles until thesputtered particles reach the substrate 360 in some cases. Within theabove range, the vertical distance between the target 300 and thesubstrate 360 is large enough to make the incident direction of thesputtered particle approximately vertical to the substrate 360, so thatdamage to the substrate 360 caused by collision of the sputteredparticles can be reduced in some cases.

FIG. 28A illustrates an example of a deposition chamber different fromthat in FIG. 27A.

The deposition chamber 301 in FIG. 28A includes a target holder 320 a, atarget holder 320 b, a backing plate 310 a, a backing plate 310 b, atarget 300 a, a target 300 b, a magnet unit 330 a, a magnet unit 330 b,a member 342, and the substrate holder 370. Note that the target 300 ais placed over the backing plate 310 a. The backing plate 310 a isplaced over the target holder 320 a. The magnet unit 330 a is placedunder the target 300 a with the backing plate 310 a positionedtherebetween. The target 300 b is placed over the backing plate 310 b.The backing plate 310 b is placed over the target holder 320 b. Themagnet unit 330 b is placed under the target 300 b with the backingplate 310 b positioned therebetween.

The magnet unit 330 a includes a magnet 330N1, a magnet 330N2, themagnet 330S, and the magnet holder 332. Note that in the magnet unit 330a, the magnet 330N1, the magnet 330N2, and the magnet 330S are placedover the magnet holder 332. The magnet 330N1, the magnet 330N2, and themagnet 330S are spaced. Note that the magnet unit 330 b has a structuresimilar to that of the magnet unit 330 a. When the substrate 360 istransferred into the deposition chamber 301, the substrate 360 is placedin contact with the substrate holder 370.

The target 300 a, the backing plate 310 a, and the target holder 320 aare separated from the target 300 b, the backing plate 310 b, and thetarget holder 320 b by the member 342. Note that the member 342 ispreferably an insulator. The member 342 may be a conductor or asemiconductor. The member 342 may be a conductor or a semiconductorwhose surface is covered with an insulator.

The target holder 320 a and the backing plate 310 a are fixed to eachother with a bolt and have the same potential. The target holder 320 ahas a function of supporting the target 300 a with the backing plate 310a positioned therebetween. The target holder 320 b and the backing plate310 b are fixed to each other with a bolt and have the same potential.The target holder 320 b has a function of supporting the target 300 bwith the backing plate 310 b positioned therebetween.

The backing plate 310 a has a function of fixing the target 300 a. Thebacking plate 310 b has a function of fixing the target 300 b.

FIG. 28A illustrates the magnetic line of force 380 a and the magneticline of force 380 b formed by the magnet unit 330 a.

The magnetic line of force 380 a is one of magnetic lines of force thatform a horizontal magnetic field in the vicinity of the target 300 a.The vicinity of the target 300 a corresponds to a region in which thevertical distance from the target 300 a is, for example, greater than orequal to 0 mm and less than or equal to 10 mm, in particular, greaterthan or equal to 0 mm and less than or equal to 5 mm.

The magnetic line of force 380 b is one of magnetic lines of force thatform a horizontal magnetic field in a plane apart from the top surfaceof the magnet unit 330 a by a vertical distance d. The vertical distanced is, for example, greater than or equal to 0 mm and less than or equalto 20 mm or greater than or equal to 5 mm and less than or equal to 15mm.

Here, with the use of the strong magnet 330N1, the strong magnet 330N2,and the strong magnet 330S, an intense magnetic field can be generatedin the vicinity of the substrate 360. Specifically, the magnetic fluxdensity of the horizontal magnetic field in the vicinity of thesubstrate 360 can be greater than or equal to 10 G and less than orequal to 100 G, preferably greater than or equal to 15 G and less thanor equal to 60 G, further preferably greater than or equal to 20 G andless than or equal to 40 G.

By setting the magnetic flux density of the magnetic field in thedeposition chamber 301 to be in the above range, an oxide semiconductorfilm with high density and high crystallinity can be deposited. Thedeposited oxide semiconductor film hardly includes plural kinds ofcrystal phases and has a substantially-single crystalline phase.

Note that the magnet unit 330 b forms a magnetic line of force similarto that formed by the magnet unit 330 a.

FIG. 28B is a top view of the magnet units 330 a and 330 b. In themagnet unit 330 a, the rectangular or substantially rectangular magnet330N1, the rectangular or substantially rectangular magnet 330N2, andthe rectangular or substantially rectangular magnet 330S are fixed tothe magnet holder 332. The magnet unit 330 a can be oscillatedhorizontally as shown in FIG. 28B. For example, the magnet unit 330 amay be oscillated with a beat of greater than or equal to 0.1 Hz andless than or equal to 1 kHz.

Thus, a region where a magnetic field on the target 300 a is intensechanges as the magnet unit 330 a is oscillated. The region with anintense magnetic field is a high-density plasma region; thus, sputteringof the target 300 a easily occurs in the vicinity of the region. Forexample, when the region with an intense magnetic field is fixed, only aspecific region of the target 300 a is used. In contrast, when themagnet unit 330 a is oscillated as shown in FIG. 28B, the target 300 acan be uniformly used. By oscillating the magnet unit 330 a, a film witha uniform thickness and a film with uniform quality can be deposited.

By oscillating the magnet unit 330 a, the state of the magnetic line offorce in the vicinity of the substrate 360 can also be changed. The sameapplies to the magnet unit 330 b.

Although the magnet unit 330 a and the magnet unit 330 b are oscillatedin this example, one embodiment of the present invention is not limitedto this example. For example, the magnet unit 330 a and the magnet unit330 b may be rotated. For example, the magnet unit 330 a and the magnetunit 330 b may be rotated with a beat of greater than or equal to 0.1 Hzand less than or equal to 1 kHz. Alternatively, the target 300 may berotated or moved. For example, the target 300 may be rotated or movedwith a beat of greater than or equal to 0.1 Hz and less than or equal to1 kHz. Further alternatively, the state of a magnetic line of force onthe top surface of the substrate 360 may be changed relatively byrotating the substrate 360. These methods may be combined.

The deposition chamber 301 may have a groove portion inside or under thebacking plate 310 a and the backing plate 310 b. By making fluid (air,nitrogen, a rare gas, water, oil, or the like) flow through the grooveportion, discharge anomaly due to an increase in the temperature of thetarget 300 a and the target 300 b or damage to the deposition chamber301 due to deformation of a component can be prevented in thesputtering. In that case, the backing plate 310 a and the target 300 aare preferably adhered to each other with a bonding member because thecooling capability is increased. Furthermore, the backing plate 310 band the target 300 b are preferably adhered to each other with a bondingmember because the cooling capability is increased.

A gasket is preferably provided between the target holder 320 a and thebacking plate 310 a, in which case an impurity is less likely to enterthe deposition chamber 301 from the outside or the groove portion. Agasket is preferably provided between the target holder 320 b and thebacking plate 310 b, in which case an impurity is less likely to enterthe deposition chamber 301 from the outside or the groove portion.

In the magnet unit 330 a, the magnets 330N1 and 330N2 and the magnet330S are placed such that their surfaces on the target 300 a side haveopposite polarities. Here, the case where the pole of each of themagnets 330N1 and 330N2 on the target 300 a side is the north pole andthe pole of the magnet 330S on the target 300 a side is the south poleis described. Note that the layout of the magnets and the poles in themagnet unit 330 a are not limited to those described here or thoseillustrated in FIG. 28A. The same applies to the magnet unit 330 b.

In the deposition, a potential whose level is varied between a highlevel and a low level is applied to the terminal V1 connected to thetarget holder 320 a and a terminal V4 connected to the target holder 320b. The potential V2 applied to the terminal V2 connected to thesubstrate holder 370 is, for example, the ground potential. A potentialV3 applied to the terminal V3 connected to the magnet holder 332 is, forexample, the ground potential. Note that the potentials applied to theterminals V1, V2, V3, and V4 are not limited to the above description.Not all the target holder 320 a, the target holder 320 b, the substrateholder 370, and the magnet holder 332 are necessarily supplied withpotentials. For example, the substrate holder 370 may be electricallyfloating. Note that the potential whose level is varied between the highlevel and the low level is applied to the terminal V1 connected to thetarget holder 320 a and the terminal V4 connected to the target holder320 b (i.e., an AC sputtering method is employed) in the exampleillustrated in FIG. 28A; however, one embodiment of the presentinvention is not limited thereto.

FIG. 28A illustrates an example where the backing plate 310 a and thetarget holder 320 a are not electrically connected to the magnet unit330 a and the magnet holder 332, but electrical connection is notlimited thereto. For example, the backing plate 310 a and the targetholder 320 a may be electrically connected to the magnet unit 330 a andthe magnet holder 332, and the backing plate 310 a, the target holder320 a, the magnet unit 330 a, and the magnet holder 332 may have thesame potential. The backing plate 310 b and the target holder 320 b arenot electrically connected to the magnet unit 330 b and the magnetholder 332 in the example, but electrical connection is not limitedthereto. For example, the backing plate 310 a and the target holder 320b may be electrically connected to the magnet unit 330 b and the magnetholder 332, and the backing plate 310 a, the target holder 320 b, themagnet unit 330 b, and the magnet holder 332 may have the samepotential.

To increase the crystallinity of the formed oxide semiconductor film,the temperature of the substrate 360 may be set high. By setting thetemperature of the substrate 360 high, migration of sputtered particlesin the vicinity of the substrate 360 can be promoted. Thus, an oxidesemiconductor film with higher density and higher crystallinity can bedeposited. Note that the temperature of the substrate 360 is, forexample, higher than or equal to room temperature and lower than 340°C., preferably higher than or equal to room temperature and lower thanor equal to 300° C., further preferably higher than or equal to 100° C.and lower than or equal to 250° C., still further preferably higher thanor equal to 100° C. and lower than or equal to 200° C.

The vertical distance between the target 300 a and the substrate 360 isgreater than or equal to 10 mm and less than or equal to 600 mm,preferably greater than or equal to 20 mm and less than or equal to 400mm, more preferably greater than or equal to 30 mm and less than orequal to 200 mm, further more preferably greater than or equal to 40 mmand less than or equal to 100 mm. Within the above range, the verticaldistance between the target 300 a and the substrate 360 is small enoughto suppress a decrease in the energy of the sputtered particles untilthe sputtered particles reach the substrate 360 in some cases. Withinthe above range, the vertical distance between the target 300 a and thesubstrate 360 is large enough to make the incident direction of thesputtered particle approximately vertical to the substrate 360, so thatdamage to the substrate 360 caused by collision of the sputteredparticles can be reduced in some cases.

The vertical distance between the target 300 b and the substrate 360 isgreater than or equal to 10 mm and less than or equal to 600 mm,preferably greater than or equal to 20 mm and less than or equal to 400mm, more preferably greater than or equal to 30 mm and less than orequal to 200 mm, further more preferably greater than or equal to 40 mmand less than or equal to 100 mm. Within the above range, the verticaldistance between the target 300 b and the substrate 360 is small enoughto suppress a decrease in the energy of the sputtered particles untilthe sputtered particles reach the substrate 360 in some cases. Withinthe above range, the vertical distance between the target 300 b and thesubstrate 360 is large enough to make the incident direction of thesputtered particle approximately vertical to the substrate 360, so thatdamage to the substrate 360 caused by collision of the sputteredparticles can be reduced in some cases.

[Facing-Targets Sputtering Apparatus (VDSP)]

Next, the facing-targets sputtering apparatus is described. FIG. 29A isa cross-sectional view of a deposition chamber of a facing-targetssputtering apparatus. The deposition chamber illustrated in FIG. 29Aincludes the target 300 a, the target 300 b, the backing plate 310 a forholding the target 300 a, the backing plate 310 b for holding the target300 b, the magnet unit 330 a placed behind the target 300 a with thebacking plate 310 a positioned therebetween, and the magnet unit 330 bplaced behind the target 300 b with the backing plate 310 b positionedtherebetween. The substrate holder 370 is placed between the target 300a and the target 300 b. The substrate 360 is transferred into thedeposition chamber, and then the substrate 360 is fixed to the substrateholder 370.

As illustrated in FIG. 29A, a power source 390 and a power source 391for applying potentials are connected to the backing plates 310 a and310 b. It is preferable to use AC power sources, which inversely applyalternate high and low potentials, as the power source 390 connected tothe backing plate 310 a and the power source 391 connected to thebacking plate 310 b. Although AC power sources are used as the powersources 390 and 391 illustrated in FIG. 29A, one embodiment of thepresent invention is not limited thereto. For example, RF power sources,DC power sources, or the like can be used as the power sources 390 and391. Alternatively, different kinds of power sources may be used as thepower sources 390 and 391.

The substrate holder 370 is preferably connected to GND. The substrateholder 370 may be in a floating state.

FIGS. 29B and 29C each show potential distribution of plasma 340 alongdashed-dotted line A-B in FIG. 29A. FIG. 29B shows the potentialdistribution in the case where a high potential is applied to thebacking plate 310 a and a low potential is applied to the backing plate310 b. In that case, a cation is accelerated toward the target 300 b.FIG. 29C shows the potential distribution in the case where a lowpotential is applied to the backing plate 310 a and a high potential isapplied to the backing plate 310 b. In that case, a cation isaccelerated toward the target 300 a. To form the oxide semiconductorfilm of one embodiment of the present invention, the state in FIG. 29Band the state in FIG. 29C are alternated.

The oxide semiconductor film of one embodiment of the present inventionis preferably formed while the plasma 340 completely reaches the surfaceof the substrate 360. For example, the substrate holder 370 and thesubstrate 360 are preferably placed in the plasma 340 as illustrated inFIG. 29A. It is particularly preferable that the substrate holder 370and the substrate 360 be placed in a positive column of the plasma 340.The positive column of the plasma 340 is, in each of FIGS. 29B and 29C,a region around the midpoint of A and B where the gradient of thepotential distribution is small. When the substrate 360 is placed in thepositive column of the plasma 340 as illustrated in FIG. 29A, thesubstrate 360 is not exposed to a high electric field portion in theplasma 340; thus, damage to the substrate 360 due to the plasma 340 canbe reduced and defects can be reduced.

It is preferable to place the substrate holder 370 and the substrate 360in the plasma 340 during deposition as illustrated in FIG. 29A alsobecause utilization efficiencies of the targets 300 a and 300 b areincreased.

As illustrated in FIG. 29A, the horizontal distance between thesubstrate holder 370 and the target 300 a is referred to as L1 and thehorizontal distance between the substrate holder 370 and the target 300b is referred to as L2. The distance L1 and the distance L2 are eachpreferably as long as the length of the substrate 360 in the horizontaldirection in FIG. 29A. In addition, it is preferable that the distancesL1 and L2 be adjusted as appropriate so that the substrate 360 is placedin the positive column of the plasma 340 as described above. Thedistances L1 and L2 can each be, for example, greater than or equal to10 mm and less than or equal to 200 mm.

In FIG. 29A, the target 300 a and the target 300 b are parallel to eachother. Moreover, the magnet unit 330 a and the magnet unit 330 b areplaced so that opposite poles face each other. Magnetic lines of forcerun from the magnet unit 330 b to the magnet unit 330 a. Therefore, inthe deposition, the plasma 340 is confined in the magnetic field formedby the magnet units 330 a and 330 b. The substrate holder 370 and thesubstrate 360 are placed in a region where the target 300 a and thetarget 300 b face each other (also referred to as a region betweentargets). Note that although the substrate holder 370 and the substrate360 are placed parallel to the direction in which the target 300 a andthe target 300 b face each other in FIG. 29A, the substrate holder 370and the substrate 360 may be inclined to the direction. By inclinationof the substrate holder 370 and the substrate 360 at 30° or more and 60°or less (typified by 45°), for example, the proportion of sputteredparticles that perpendicularly reach the substrate 360 during depositioncan be increased.

The structure illustrated in FIG. 30 is different from that illustratedin FIG. 29A in that the target 300 a and the target 300 b that face eachother are not parallel but inclined to each other (in V-shape). Thus,the description for FIG. 29A is referred to for the description exceptfor the positions of the targets. The magnet unit 330 a and the magnetunit 330 b are placed so that opposite poles face each other. Thesubstrate holder 370 and the substrate 360 are placed in the regionbetween targets. With the targets 300 a and 300 b placed as illustratedin FIG. 30 , the proportion of sputtered particles that reach thesubstrate 360 can be increased; accordingly, the deposition rate can beincreased.

The positions of the substrate holder 370 and the substrate 360 are notlimited to in the plasma 340 as illustrated in FIG. 29A. The substrateholder 370 and the substrate 360 may be placed outside the plasma 340 asillustrated in FIG. 31 , for example. In that case, the substrate 360 isnot exposed to a high electric field region of the plasma 340, leadingto a reduction in damage due to the plasma 340. Note that theutilization efficiencies of the targets 300 a and 300 b are decreased asthe distance between the plasma 340 and the substrate 360 is increased.It is preferable that the position of the substrate holder 370 beadjustable as illustrated in FIG. 31 .

The substrate holder 370 may be placed either above or below the regionbetween targets. Alternatively, the substrate holders 370 may be placedabove and below the region. When the substrate holders 370 are providedabove and below the region, deposition on two or more substrates can beperformed at once, leading to an increase in productivity.

The facing-targets sputtering apparatus can stably generate plasma evenin a high vacuum. Thus, deposition can be performed at a pressure higherthan or equal to 0.005 Pa and lower than or equal to 0.09 Pa, forexample. As a result, the concentration of impurities contained duringdeposition can be reduced.

The use of the facing-targets sputtering apparatus allows deposition ina high vacuum or deposition with less plasma damage and thus can providea film with high crystallinity even when the temperature of thesubstrate 360 is low (e.g., higher than or equal to room temperature andlower than 100° C.).

FIG. 32A illustrates another example of a facing-targets sputteringapparatus.

FIG. 32A is a schematic cross-sectional view of a deposition chamber ofa facing-targets sputtering apparatus. Unlike in the deposition chamberillustrated in FIG. 29A, a target shield 322 and a target shield 323 areprovided. The power source 391 connected to the backing plates 310 a and310 b is also provided.

The target shields 322 and 323 are connected to GND as illustrated inFIG. 32A. This means that the plasma 340 is generated by a potentialdifference between the backing plates 310 a and 310 b to which apotential of the power source 391 is applied and the target shields 322and 323 to which GND is applied.

The oxide semiconductor film of one embodiment of the present inventionis preferably formed while the plasma 340 completely reaches the surfaceof the substrate 360. For example, the substrate holder 370 and thesubstrate 360 are preferably placed in the plasma 340 as illustrated inFIG. 32A. It is particularly preferable that the substrate holder 370and the substrate 360 be placed in a positive column of the plasma 340.The positive column of the plasma is a region where the gradient of thepotential distribution is small. When the substrate 360 is placed in thepositive column of the plasma 340 as illustrated in FIG. 32A, thesubstrate 360 is not exposed to a high electric field portion in theplasma 340; thus, damage to the substrate 360 due to the plasma 340 canbe reduced and an oxide with a favorable film quality can be obtained.

It is preferable to place the substrate holder 370 and the substrate 360in the plasma 340 during deposition as illustrated in FIG. 32A alsobecause utilization efficiencies of the targets 300 a and 300 b areincreased.

As illustrated in FIG. 32A, the horizontal distance between thesubstrate holder 370 and the target 300 a is referred to as L1 and thehorizontal distance between the substrate holder 370 and the target 300b is referred to as L2. The distance L1 and the distance L2 are eachpreferably as long as the length of the substrate 360 in the horizontaldirection in FIG. 32A. In addition, it is preferable that the distancesL1 and L2 be adjusted as appropriate so that the substrate 360 is placedin the positive column of the plasma 340 as described above.

The positions of the substrate holder 370 and the substrate 360 are notlimited to in the plasma 340 as illustrated in FIG. 32A. The substrateholder 370 and the substrate 360 may be placed outside the plasma 340 asillustrated in FIG. 32B, for example. In that case, the substrate 360 isnot exposed to a high electric field region of the plasma 340, leadingto a reduction in damage due to the plasma 340. Note that theutilization efficiencies of the targets 300 a and 300 b are decreased asthe distance between the plasma 340 and the substrate 360 are increased.It is preferable that the position of the substrate holder 370 beadjustable as in FIG. 32B.

The substrate holder 370 may be placed above a region between targets asillustrated in FIG. 32B, or may be placed below the region.Alternatively, the substrate holders 370 may be placed above and belowthe region. Providing the substrate holders 370 above and below theregion allows deposition on two or more substrates at once, leading toan increase in productivity.

In the above-described facing-targets sputtering apparatuses, plasma isconfined by magnetic fields between targets; thus, plasma damage to asubstrate can be reduced. Furthermore, a deposited film can haveimproved step coverage because an incident angle of a sputtered particleto a substrate can be made smaller by the inclination of the target.Moreover, deposition in a high vacuum enables the concentration ofimpurities contained in the film to be reduced.

Note that a parallel-plate-type sputtering apparatus or an ion beamsputtering apparatus may be provided in the deposition chamber.

<3-2. Deposition Apparatus>

A deposition apparatus including a deposition chamber with which theoxide semiconductor film of one embodiment of the present invention canbe formed is described below.

First, a structure of a deposition apparatus which allows the entry offew impurities into a film at the time of the deposition or the like isdescribed with reference to FIG. 33 and FIGS. 34A to 34C.

FIG. 33 is a top view schematically illustrating a single wafermulti-chamber deposition apparatus 2700. The deposition apparatus 2700includes an atmosphere-side substrate supply chamber 2701 including acassette port 2761 for holding a substrate and an alignment port 2762for performing alignment of a substrate, an atmosphere-side substratetransfer chamber 2702 through which a substrate is transferred from theatmosphere-side substrate supply chamber 2701, a load lock chamber 2703a where a substrate is carried and the pressure inside the chamber isswitched from atmospheric pressure to reduced pressure or from reducedpressure to atmospheric pressure, an unload lock chamber 2703 b where asubstrate is carried out and the pressure inside the chamber is switchedfrom reduced pressure to atmospheric pressure or from atmosphericpressure to reduced pressure, a transfer chamber 2704 through which asubstrate is transferred in a vacuum, a substrate heating chamber 2705where a substrate is heated, and deposition chambers 2706 a, 2706 b, and2706 c in each of which a target is placed for deposition. Note that forthe deposition chambers 2706 a, 2706 b, and 2706 c, the structure of thedeposition chamber described above can be referred to.

The atmosphere-side substrate transfer chamber 2702 is connected to theload lock chamber 2703 a and the unload lock chamber 2703 b, the loadlock chamber 2703 a and the unload lock chamber 2703 b are connected tothe transfer chamber 2704, and the transfer chamber 2704 is connected tothe substrate heating chamber 2705 and the deposition chambers 2706 a,2706 b, and 2706 c.

Gate valves 2764 are provided for connecting portions between chambersso that each chamber except the atmosphere-side substrate supply chamber2701 and the atmosphere-side substrate transfer chamber 2702 can beindependently kept under vacuum. Moreover, the atmosphere-side substratetransfer chamber 2702 and the transfer chamber 2704 each include atransfer robot 2763, with which a substrate can be transferred.

Furthermore, it is preferable that the substrate heating chamber 2705also serve as a plasma treatment chamber. In the deposition apparatus2700, it is possible to transfer a substrate without exposure to the airbetween treatment and treatment; therefore, adsorption of impurities ona substrate can be suppressed. In addition, the order of deposition,heat treatment, or the like can be freely determined. Note that thenumber of the transfer chambers, the number of the deposition chambers,the number of the load lock chambers, the number of the unload lockchambers, and the number of the substrate heating chambers are notlimited to the above, and the numbers thereof can be set as appropriatedepending on the space for placement or the process conditions.

Next, FIG. 34A, FIG. 34B, and FIG. 34C are a cross-sectional view takenalong dashed-dotted line V1-V2, a cross-sectional view taken alongdashed-dotted line W1-W2, and a cross-sectional view taken alongdashed-dotted line W2-W3, respectively, in the deposition apparatus 2700illustrated in FIG. 33 .

FIG. 34A is a cross section of the substrate heating chamber 2705 andthe transfer chamber 2704, and the substrate heating chamber 2705includes a plurality of heating stages 2765 which can hold a substrate.Furthermore, the substrate heating chamber 2705 is connected to a vacuumpump 2770 through a valve. As the vacuum pump 2770, a dry pump and amechanical booster pump can be used, for example.

As a heating mechanism which can be used for the substrate heatingchamber 2705, a resistance heater may be used for heating, for example.Alternatively, heat conduction or heat radiation from a medium such as aheated gas may be used as the heating mechanism. For example, RTA suchas gas rapid thermal annealing (GRTA) or lamp rapid thermal annealing(LRTA) can be used. The LRTA is a method for heating an object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. Inthe GRTA, heat treatment is performed using a high-temperature gas. Aninert gas is used as the gas.

Moreover, the substrate heating chamber 2705 is connected to a refiner2781 through a mass flow controller 2780. Note that although the massflow controller 2780 and the refiner 2781 can be provided for each of aplurality of kinds of gases, only one mass flow controller 2780 and onerefiner 2781 are provided for easy understanding. As the gas introducedto the substrate heating chamber 2705, a gas whose dew point is −80° C.or lower, preferably −100° C. or lower can be used; for example, anoxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) areused.

The transfer chamber 2704 includes the transfer robot 2763. The transferrobot 2763 can transfer a substrate to each chamber. Furthermore, thetransfer chamber 2704 is connected to the vacuum pump 2770 and acryopump 2771 through valves. With such a structure, exhaust can beperformed using the vacuum pump 2770 when the pressure inside thetransfer chamber 2704 is in the range of atmospheric pressure to low ormedium vacuum (about several hundred pascals to 0.1 Pa) and then, byswitching the valves, exhaust can be performed using the cryopump 2771when the pressure inside the transfer chamber 2704 is in the range ofmiddle vacuum to high or ultra-high vacuum (approximately 0.1 Pa to1×10⁻⁷ Pa).

Alternatively, two or more cryopumps 2771 may be connected in parallelto the transfer chamber 2704. With such a structure, even when one ofthe cryopumps is in regeneration, exhaust can be performed using any ofthe other cryopumps. Note that the above regeneration refers totreatment for discharging molecules (or atoms) entrapped in thecryopump. When molecules (or atoms) are entrapped too much in acryopump, the exhaust capability of the cryopump is lowered; therefore,regeneration is performed regularly.

FIG. 34B is a cross section of the deposition chamber 2706 b, thetransfer chamber 2704, and the load lock chamber 2703 a.

Here, the details of the deposition chamber (sputtering chamber) aredescribed with reference to FIG. 34B. In the deposition chamber 2706 billustrated in FIG. 34B, a target 2766 a, a target 2766 b, a targetshield 2767 a, a target shield 2767 b, a magnet unit 2790 a, a magnetunit 2790 b, a substrate holder 2768, and power sources 2791 areprovided. Although not illustrated, the targets 2766 a and 2766 b areeach fixed to a target holder with a backing plate positionedtherebetween. The targets 2766 a and 2766 b are electrically connectedto the power sources 2791. The magnet units 2790 a and 2790 b are placedbehind the targets 2766 a and 2766 b, respectively. The target shields2767 a and 2767 b are placed to surround end portions of the targets2766 a and 2766 b, respectively. Note that here, the substrate 2769 issupported by the substrate holder 2768. The substrate holder 2768 isfixed to the deposition chamber 2706 b by an movable member 2784. Owingto the movable member 2784, the substrate holder 2768 can move to aregion between the targets 2766 a and 2766 b (region between targets).Providing the substrate holder 2768 supporting the substrate 2769 in theregion between targets can reduce damage due to plasma in some cases,for example. Although not illustrated, the substrate holder 2768 mayinclude a substrate holding mechanism which holds the substrate 2769, arear heater which heats the substrate 2769 from the back surface, or thelike.

The target shields 2767 a and 2767 b can suppress deposition of aparticle which is sputtered from the targets 2766 a and 2766 b on aregion where deposition is not needed. Moreover, the target shields 2767a and 2767 b are preferably processed to prevent accumulated sputteredparticles from being separated. For example, blasting treatment whichincreases surface roughness may be performed, or roughness may be formedon the surface of the target shields 2767 a and 2767 b.

The deposition chamber 2706 b is connected to the mass flow controller2780 through a gas heating system 2782, and the gas heating system 2782is connected to the refiner 2781 through the mass flow controller 2780.With the gas heating system 2782, a gas which is introduced to thedeposition chamber 2706 b can be heated to a temperature higher than orequal to 40° C. and lower than or equal to 400° C., preferably higherthan or equal to 50° C. and lower than or equal to 200° C. Note thatalthough the gas heating system 2782, the mass flow controller 2780, andthe refiner 2781 can be provided for each of a plurality of kinds ofgases, only one gas heating system 2782, one mass flow controller 2780,and one refiner 2781 are provided for easy understanding. As the gasintroduced to the deposition chamber 2706 b, a gas whose dew point is−80° C. or lower, preferably −100° C. or lower can be used; for example,an oxygen gas, a nitrogen gas, and a rare gas (e.g., an argon gas) areused.

In the case where the refiner is provided near a gas inlet, the lengthof a pipe between the refiner and the deposition chamber 2706 b is lessthan or equal to 10 m, preferably less than or equal to 5 m, morepreferably less than or equal to 1 m. When the length of the pipe isless than or equal to 10 m, less than or equal to 5 m, or less than orequal to 1 m, the effect of the release of gas from the pipe can bereduced accordingly. As the pipe for the gas, a metal pipe the inside ofwhich is covered with iron fluoride, aluminum oxide, chromium oxide, orthe like is preferably used. With the above pipe, the amount of releasedgas containing impurities is made small and the entry of impurities intothe gas can be reduced as compared with a SUS316L-EP pipe, for example.Furthermore, a high-performance ultra-compact metal gasket joint (UPGjoint) may be used as a joint of the pipe. A structure where all thematerials of the pipe are metals is preferable because the effect of thegenerated released gas or the external leakage can be reduced ascompared with a structure where a resin or the like is used.

The deposition chamber 2706 b is connected to a turbo molecular pump2772 and the vacuum pump 2770 through valves.

In addition, the deposition chamber 2706 b is provided with a cryotrap2751.

The cryotrap 2751 is a mechanism which can adsorb a molecule (or anatom) having a relatively high melting point, such as water. The turbomolecular pump 2772 is capable of stably removing a large-sized molecule(or atom), needs low frequency of maintenance, and thus enables highproductivity, whereas it has a low capability in removing hydrogen andwater. Hence, the cryotrap 2751 is connected to the deposition chamber2706 b so as to have a high capability in removing water or the like.The temperature of a refrigerator of the cryotrap 2751 is set to belower than or equal to 100 K, preferably lower than or equal to 80 K. Inthe case where the cryotrap 2751 includes a plurality of refrigerators,it is preferable to set the temperatures of the refrigerators atdifferent temperatures because efficient exhaust is possible. Forexample, the temperature of a first-stage refrigerator may be set to belower than or equal to 100 K and the temperature of a second-stagerefrigerator may be set to be lower than or equal to 20 K. Note thatwhen a titanium sublimation pump is used instead of the cryotrap, ahigher vacuum can be achieved in some cases. Using an ion pump insteadof a cryopump or a turbo molecular pump can also achieve a higher vacuumin some cases.

Note that the exhaust method of the deposition chamber 2706 b is notlimited to the above, and a structure similar to that in the exhaustmethod described above for the transfer chamber 2704 (the exhaust methodusing the cryopump and the vacuum pump) may be employed. Needless tosay, the exhaust method of the transfer chamber 2704 may have astructure similar to that of the deposition chamber 2706 b (the exhaustmethod using the turbo molecular pump and the vacuum pump).

Note that in each of the transfer chamber 2704, the substrate heatingchamber 2705, and the deposition chamber 2706 b which are describedabove, the back pressure (total pressure) and the partial pressure ofeach gas molecule (atom) are preferably set as follows. In particular,the back pressure and the partial pressure of each gas molecule (atom)in the deposition chamber 2706 b need to be noted because impuritiesmight enter a film to be formed.

In each of the above chambers, the back pressure (total pressure) isless than or equal to 1×10⁻⁴ Pa, preferably less than or equal to 3×10⁻⁵Pa, more preferably less than or equal to 1×10⁻⁵ Pa. In each of theabove chambers, the partial pressure of a gas molecule (atom) having amass-to-charge ratio (m/z) of 18 is less than or equal to 3×10⁻⁵ Pa,preferably less than or equal to 1×10⁻⁵ Pa, more preferably less than orequal to 3×10⁻⁶ Pa. Moreover, in each of the above chambers, the partialpressure of a gas molecule (atom) having a mass-to-charge ratio (m/z) of28 is less than or equal to 3×10⁻⁵ Pa, preferably less than or equal to1×10⁻⁵ Pa, more preferably less than or equal to 3×10⁻⁶ Pa. Furthermore,in each of the above chambers, the partial pressure of a gas molecule(atom) having a mass-to-charge ratio (m/z) of 44 is less than or equalto 3×10⁻⁵ Pa, preferably less than or equal to 1×10⁻⁵ Pa, morepreferably less than or equal to 3×10⁻⁶ Pa.

Note that a total pressure and a partial pressure in a vacuum chambercan be measured using a mass analyzer. For example, Qulee CGM-051, aquadrupole mass analyzer (also referred to as Q-mass) manufactured byULVAC, Inc. may be used.

Moreover, the transfer chamber 2704, the substrate heating chamber 2705,and the deposition chamber 2706 b which are described above preferablyhave a small amount of external leakage or internal leakage.

For example, in each of the transfer chamber 2704, the substrate heatingchamber 2705, and the deposition chamber 2706 b which are describedabove, the leakage rate is less than or equal to 3×10⁻⁶ Pa·m³/s,preferably less than or equal to 1×10⁻⁶ Pa·m³/s. The leakage rate of agas molecule (atom) having a mass-to-charge ratio (m/z) of 18 is lessthan or equal to 1×10⁻⁷ Pa·m³/s, preferably less than or equal to 3×10⁻⁸Pa·m³/s. The leakage rate of a gas molecule (atom) having amass-to-charge ratio (m/z) of 28 is less than or equal to 1×10⁻⁵Pa·m³/s, preferably less than or equal to 1×10⁻⁶ Pa·m³/s. The leakagerate of a gas molecule (atom) having a mass-to-charge ratio (m/z) of 44is less than or equal to 3×10⁻⁶ Pa·m³/s, preferably less than or equalto 1×10⁻⁶ Pa·m³/s.

Note that a leakage rate can be derived from the total pressure andpartial pressure measured using the mass analyzer.

The leakage rate depends on external leakage and internal leakage. Theexternal leakage refers to inflow of gas from the outside of a vacuumsystem through a minute hole, a sealing defect, or the like. Theinternal leakage is due to leakage through a partition, such as a valve,in a vacuum system or due to released gas from an internal member.Measures need to be taken from both aspects of external leakage andinternal leakage in order that the leakage rate can be set to be lessthan or equal to the above value.

For example, an open/close portion of the deposition chamber 2706 b ispreferably sealed with a metal gasket. For the metal gasket, metalcovered with iron fluoride, aluminum oxide, or chromium oxide ispreferably used. The metal gasket realizes higher adhesion than anO-ring, and can reduce the external leakage. Furthermore, with the useof the metal covered with iron fluoride, aluminum oxide, chromium oxide,or the like, which is in the passive state, the release of gascontaining impurities released from the metal gasket is suppressed, sothat the internal leakage can be reduced.

For a member of the deposition apparatus 2700, aluminum, chromium,titanium, zirconium, nickel, or vanadium, which releases a smalleramount of gas containing impurities, is used. Alternatively, for theabove member, an alloy containing iron, chromium, nickel, and the likecovered with the above material may be used. The alloy containing iron,chromium, nickel, and the like is rigid, resistant to heat, and suitablefor processing. Here, when surface unevenness of the member is decreasedby polishing or the like to reduce the surface area, the release of gascan be reduced.

Alternatively, the above member of the deposition apparatus 2700 may becovered with iron fluoride, aluminum oxide, chromium oxide, or the like.

The member of the deposition apparatus 2700 is preferably formed usingonly metal when possible. For example, in the case where a viewingwindow formed with quartz or the like is provided, it is preferable thatthe surface of the viewing window be thinly covered with iron fluoride,aluminum oxide, chromium oxide, or the like so as to suppress release ofgas.

When an adsorbed substance is present in the deposition chamber, theadsorbed substance does not affect the pressure in the depositionchamber because it is adsorbed onto an inner wall or the like; however,the adsorbed substance causes gas to be released when the inside of thedeposition chamber is evacuated. Therefore, although there is nocorrelation between the leakage rate and the exhaust rate, it isimportant that the adsorbed substance present in the deposition chamberbe desorbed as much as possible and exhaust be performed in advance withthe use of a pump with high exhaust capability. Note that the depositionchamber may be subjected to baking to promote desorption of the adsorbedsubstance. By the baking, the desorption rate of the adsorbed substancecan be increased about tenfold. The baking can be performed at atemperature in the range of 100° C. to 450° C. At this time, when theadsorbed substance is removed while an inert gas is introduced to thedeposition chamber, the desorption rate of water or the like, which isdifficult to be desorbed simply by exhaust, can be further increased.Note that when the inert gas which is introduced is heated tosubstantially the same temperature as the baking temperature, thedesorption rate of the adsorbed substance can be further increased.Here, a rare gas is preferably used as an inert gas. Depending on thekind of a film to be deposited, oxygen or the like may be used insteadof an inert gas. For example, in deposition of an oxide, the use ofoxygen which is the main component of the oxide is preferable in somecases. The baking is preferably performed using a lamp.

Alternatively, treatment for evacuating the inside of the depositionchamber is preferably performed a certain period of time after heatedoxygen, a heated inert gas such as a heated rare gas, or the like isintroduced to increase a pressure in the deposition chamber. Theintroduction of the heated gas can desorb the adsorbed substance in thedeposition chamber, and the impurities present in the deposition chambercan be reduced. Note that an advantageous effect can be achieved whenthis treatment is repeated more than or equal to 2 times and less thanor equal to 30 times, preferably more than or equal to 5 times and lessthan or equal to 15 times. Specifically, an inert gas, oxygen, or thelike with a temperature higher than or equal to 40° C. and lower than orequal to 400° C., preferably higher than or equal to 50° C. and lowerthan or equal to 200° C. is introduced to the deposition chamber, sothat the pressure therein can be kept to be greater than or equal to 0.1Pa and less than or equal to 10 kPa, preferably greater than or equal to1 Pa and less than or equal to 1 kPa, more preferably greater than orequal to 5 Pa and less than or equal to 100 Pa in the time range of 1minute to 300 minutes, preferably 5 minutes to 120 minutes. After that,the inside of the deposition chamber is evacuated in the time range of 5minutes to 300 minutes, preferably 10 minutes to 120 minutes.

The desorption rate of the adsorbed substance can be further increasedalso by dummy deposition. Here, the dummy deposition refers todeposition on a dummy substrate by a sputtering method or the like, inwhich a film is deposited on the dummy substrate and the inner wall ofthe deposition chamber so that impurities in the deposition chamber andan adsorbed substance on the inner wall of the deposition chamber areconfined in the film. For a dummy substrate, a substrate which releasesa smaller amount of gas is preferably used. By performing dummydeposition, the concentration of impurities in a film to be formed latercan be reduced. Note that the dummy deposition may be performed at thesame time as the baking of the deposition chamber.

Next, the details of the transfer chamber 2704 and the load lock chamber2703 a illustrated in FIG. 34B and the atmosphere-side substratetransfer chamber 2702 and the atmosphere-side substrate supply chamber2701 illustrated in FIG. 34C are described. Note that FIG. 34C is across section of the atmosphere-side substrate transfer chamber 2702 andthe atmosphere-side substrate supply chamber 2701.

For the transfer chamber 2704 illustrated in FIG. 34B, the descriptionof the transfer chamber 2704 illustrated in FIG. 34A can be referred to.

The load lock chamber 2703 a includes a substrate delivery stage 2752.When a pressure in the load lock chamber 2703 a becomes atmosphericpressure by being increased from reduced pressure, the substratedelivery stage 2752 receives a substrate from the transfer robot 2763provided in the atmosphere-side substrate transfer chamber 2702. Afterthat, the load lock chamber 2703 a is evacuated into vacuum so that thepressure therein becomes reduced pressure and then the transfer robot2763 provided in the transfer chamber 2704 receives the substrate fromthe substrate delivery stage 2752.

Furthermore, the load lock chamber 2703 a is connected to the vacuumpump 2770 and the cryopump 2771 through valves. For a method forconnecting exhaust systems such as the vacuum pump 2770 and the cryopump2771, the description of the method for connecting the transfer chamber2704 can be referred to, and the description thereof is omitted here.Note that the unload lock chamber 2703 b illustrated in FIG. 33 can havea structure similar to that in the load lock chamber 2703 a.

The atmosphere-side substrate transfer chamber 2702 includes thetransfer robot 2763. The transfer robot 2763 can deliver a substratefrom the cassette port 2761 to the load lock chamber 2703 a or deliver asubstrate from the load lock chamber 2703 a to the cassette port 2761.Furthermore, a mechanism for cleaning dust or a particle, such as highefficiency particulate air (HEPA) filter, may be provided above theatmosphere-side substrate transfer chamber 2702 and the atmosphere-sidesubstrate supply chamber 2701.

The atmosphere-side substrate supply chamber 2701 includes a pluralityof cassette ports 2761. The cassette port 2761 can hold a plurality ofsubstrates.

The surface temperature of the target is set to be lower than or equalto 100° C., preferably lower than or equal to 50° C., more preferablyabout room temperature (typically, 25° C.). In a sputtering apparatusfor a large substrate, a large target is often used. However, it isdifficult to form a target for a large substrate without a juncture. Infact, a plurality of targets are arranged so that there is as littlespace as possible therebetween to obtain a large shape; however, aslight space is inevitably generated. When the surface temperature ofthe target increases, in some cases, zinc or the like is volatilizedfrom such a slight space and the space might be expanded gradually. Whenthe space expands, a metal of a backing plate or a metal contained in abonding member used for adhesion between the backing plate and a targetmight be sputtered and might cause an increase in impurityconcentration. Thus, it is preferable that the target be cooledsufficiently.

Specifically, for the backing plate, a metal having high conductivityand a high heat dissipation property (specifically copper) is used. Thetarget can be cooled efficiently by making a sufficient amount ofcooling water flow through a groove portion which is formed in thebacking plate.

With the above-described deposition apparatus, an oxide semiconductorfilm whose hydrogen concentration measured by SIMS is lower than 1×10²⁰atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, further preferablylower than 5×10¹⁸ atoms/cm³, still further preferably lower than 1×10¹⁸atoms/cm³ can be formed.

Furthermore, an oxide semiconductor film whose nitrogen concentrationmeasured by SIMS is lower than or equal to 5×10¹⁸ atoms/cm³, preferablylower than or equal to 1×10¹⁸ atoms/cm³ can be formed.

Furthermore, an oxide semiconductor film whose carbon concentrationmeasured by SIMS is lower than or equal to 2×10¹⁸ atoms/cm³, preferablylower than or equal to 2×10¹⁷ atoms/cm³ can be formed.

The oxide semiconductor film having small amounts of impurities andoxygen vacancies is an oxide semiconductor film with low carrier density(specifically, lower than 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³,further preferably lower than 1×10¹⁰/cm³, and is higher than or equal to1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor. The oxide semiconductor film has a low impurityconcentration and a low density of defect states. Thus, the oxidesemiconductor film can be referred to as an oxide semiconductor filmhaving stable characteristics.

Furthermore, an oxide semiconductor can be deposited in which thereleased amount of each of the following gas molecules (atoms) measuredby TDS is less than or equal to 1×10¹⁹/cm³, preferably less than orequal to 1×10¹⁸/cm³: a gas molecule (atom) having a mass-to-charge ratio(m/z) of 2 (e.g., a hydrogen molecule), a gas molecule (atom) having amass-to-charge ratio (m/z) of 18, a gas molecule (atom) having amass-to-charge ratio (m/z) of 28, and a gas molecule (atom) having amass-to-charge ratio (m/z) of 44.

With the use of the above deposition apparatus, the entry of impuritiesinto the oxide semiconductor film can be suppressed. Furthermore, when afilm in contact with the oxide semiconductor film is formed with the useof the above deposition apparatus, the entry of impurities into theoxide semiconductor film from the film in contact therewith can besuppressed.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments and the examples.

Embodiment 4

In this embodiment, a display device that includes a semiconductordevice of one embodiment of the present invention is described withreference to FIGS. 35A to 35C.

<4. Display Device>

The display device illustrated in FIG. 35A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 502), a circuit portion provided outside the pixel portion502 and including a circuit for driving the pixels (hereinafter theportion is referred to as a driver circuit portion 504), circuits eachhaving a function of protecting an element (hereinafter the circuits arereferred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

A part or the whole of the driver circuit portion 504 is preferablyformed over a substrate over which the pixel portion 502 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 504 isnot formed over the substrate over which the pixel portion 502 isformed, the part or the whole of the driver circuit portion 504 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter, suchcircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as scan lines GL_1 to GL_X).Note that a plurality of gate drivers 504 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the gate driver504 a has a function of supplying an initialization signal. Withoutbeing limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (image signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 whichis based on the image signal. In addition, the source driver 504 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Furthermore, the source driver 504 b has a function of controllingthe potentials of wirings supplied with data signals (hereinafter suchwirings are referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 504 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 504 b cansupply another signal.

The source driver 504 b includes a plurality of analog switches or thelike, for example. The source driver 504 b can output, as the datasignals, signals obtained by time-dividing the image signal bysequentially turning on the plurality of analog switches. The sourcedriver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 504 a through the scanline GL_m, and a data signal is input from the source driver 504 bthrough the data line DL_n in accordance with the potential of the scanline GL_m.

The protection circuit 506 shown in FIG. 35A is connected to, forexample, the scan line GL between the gate driver 504 a and the pixelcircuit 501. Alternatively, the protection circuit 506 is connected tothe data line DL between the source driver 504 b and the pixel circuit501. Alternatively, the protection circuit 506 can be connected to awiring between the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 means a portion having terminals for inputtingpower, control signals, and image signals to the display device fromexternal circuits.

The protection circuit 506 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 35A, the protection circuits 506 are provided forthe pixel portion 502 and the driver circuit portion 504, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 506 is not limited to that, andfor example, the protection circuit 506 may be configured to beconnected to the gate driver 504 a or the protection circuit 506 may beconfigured to be connected to the source driver 504 b. Alternatively,the protection circuit 506 may be configured to be connected to theterminal portion 507.

In FIG. 35A, an example in which the driver circuit portion 504 includesthe gate driver 504 a and the source driver 504 b is shown; however, thestructure is not limited thereto. For example, only the gate driver 504a may be formed and a separately prepared substrate where a sourcedriver circuit is formed (e.g., a driver circuit substrate formed with asingle crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 35A can have thestructure illustrated in FIG. 35B, for example.

The pixel circuit 501 illustrated in FIG. 35B includes a liquid crystalelement 570, a transistor 550, and a capacitor 560. As the transistor550, any of the transistors described in the above embodiments can beused.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set in accordance with the specifications of the pixelcircuit 501 as appropriate. The alignment state of the liquid crystalelement 570 depends on written data. A common potential may be suppliedto one of the pair of electrodes of the liquid crystal element 570included in each of the plurality of pixel circuits 501. Furthermore,the potential supplied to one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 in one row may be differentfrom the potential supplied to one of the pair of electrodes of theliquid crystal element 570 in the pixel circuit 501 in another row.

As a driving method of the display device including the liquid crystalelement 570, any of the following modes can be used, for example: atwisted nematic (TN) mode, a super-twisted nematic (STN) mode, avertical alignment (VA) mode, a multi-domain vertical alignment (MVA)mode, a patterned vertical alignment (PVA) mode, an in-plane-switching(IPS) mode, a fringe field switching (FFS) mode, an axially symmetricaligned micro-cell (ASM) mode, an optically compensated birefringence(OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, a transverse bendalignment (TBA) mode, and the like.

Other examples of the driving method of the display device include anelectrically controlled birefringence (ECB) mode, a polymer dispersedliquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC)mode, and a guest-host mode. Note that the present invention is notlimited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 has afunction of controlling whether to write a data signal by being turnedon or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set in accordance with the specifications of the pixel circuit 501as appropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 501 inFIG. 35B, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 35A, whereby the transistors550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 35Acan have the structure illustrated in FIG. 35C, for example.

The pixel circuit 501 illustrated in FIG. 35C includes transistors 552and 554, a capacitor 562, and a light-emitting element 572. Any of thetransistors described in the above embodiments can be used as one orboth of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a data line DL_n). A gate electrode of thetransistor 552 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 554 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 inFIG. 35C, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 35A, whereby the transistors552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

Although the structures including the liquid crystal element 570 or thelight-emitting element 572 as a display element of the display deviceare described in this embodiment, one embodiment of the presentinvention is not limited to these structures and a variety of elementsmay be included in the display device.

For example, the display device includes at least one of a liquidcrystal element, an EL element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), a displayelement using micro electro mechanical systems (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, a display element using a carbon nanotube, and the like.Alternatively, the display device may include a display medium whosecontrast, luminance, reflectivity, transmittance, or the like is changedby electrical or magnetic effect. Examples of display devices includingelectron emitters are a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atranflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atranflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes. Thus,the power consumption can be further reduced.

A progressive type display, an interlace type display, or the like canbe employed as the display type of the display device of thisembodiment. Furthermore, color elements controlled in a pixel at thetime of color display are not limited to three colors: R, G, and B (R,G, and B correspond to red, green, and blue, respectively). For example,four pixels of the R pixel, the G pixel, the B pixel, and a W (white)pixel may be included. Alternatively, a color element may be composed oftwo colors among R, G, and B as in PenTile layout. The two colors maydiffer among color elements. Alternatively, one or more colors ofyellow, cyan, magenta, and the like may be added to RGB. Furthermore,the size of a display region may be different depending on respectivedots of the color elements. Embodiments of the disclosed invention arenot limited to a display device for color display; the disclosedinvention can also be applied to a display device for monochromedisplay.

White light (W) may be emitted from a backlight (e.g., an organic ELelement, an inorganic EL element, an LED, or a fluorescent lamp) in thedisplay device. Furthermore, a coloring layer (also referred to as acolor filter) may be provided in the display device. As the coloringlayer, red (R), green (G), blue (B), yellow (Y), or the like may becombined as appropriate, for example. With the use of the coloringlayer, higher color reproducibility can be obtained than in the casewithout the coloring layer. In this case, by providing a region with thecoloring layer and a region without the coloring layer, white light inthe region without the coloring layer may be directly utilized fordisplay. By partly providing the region without the coloring layer, adecrease in luminance due to the coloring layer can be suppressed, and20% to 30% of power consumption can be reduced in some cases when animage is displayed brightly. Note that in the case where full-colordisplay is performed using self-luminous elements such as organic ELelements or inorganic EL elements, the elements may emit light of theirrespective colors R, G, B, Y, and W. By using self-luminous elements,power consumption can be further reduced as compared to the case ofusing the coloring layer in some cases.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments and the examples.

Embodiment 5

In this embodiment, a display device including a semiconductor device ofone embodiment of the present invention and an electronic device inwhich the display device is provided with an input device will bedescribed with reference to FIGS. 36A and 36B, FIGS. 37A and 37B, FIG.38 , FIGS. 39A and 39B, FIGS. 40A and 40B, and FIG. 41 .

<5-1. Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device will be described as an example of an electronic device. Inaddition, an example in which a touch sensor is used as an input devicewill be described.

FIGS. 36A and 36B are perspective views of the touch panel 2000. Notethat FIGS. 36A and 36B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 36B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 36B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used, forexample. Examples of the capacitive touch sensor are a surfacecapacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self-capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of a mutual capacitive type is preferablebecause multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 36B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 36A and 36B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

Note that as a material of the conductive films used for the electrodes2591, the electrodes 2592, and the wirings 2598, that is, wirings andelectrodes forming the touch panel, a transparent conductive filmcontaining indium oxide, tin oxide, zinc oxide, or the like (e.g., ITO)can be given. For example, a low-resistance material is preferably usedas a material that can be used as the wirings and electrodes forming thetouch panel. For example, silver, copper, aluminum, a carbon nanotube,graphene, or a metal halide (such as a silver halide) may be used.Alternatively, a metal nanowire including a plurality of conductors withan extremely small width (for example, a diameter of several nanometers)may be used. Further alternatively, a net-like metal mesh with aconductor may be used. For example, an Ag nanowire, a Cu nanowire, an Alnanowire, an Ag mesh, a Cu mesh, or an Al mesh may be used. For example,in the case of using an Ag nanowire as the wirings and electrodesforming the touch panel, a visible light transmittance of 89% or moreand a sheet resistance of 40 Ω/cm² or more and 100 Ω/cm² or less can beachieved. Since the above-described metal nanowire, metal mesh, carbonnanotube, graphene, and the like, which are examples of the materialthat can be used as the wirings and electrodes forming the touch panel,have high visible light transmittances, they may be used as electrodesof display elements (e.g., a pixel electrode or a common electrode).

<5-2. Display Device>

Next, the display device 2501 will be described in detail with referenceto FIGS. 37A and 37B. FIGS. 37A and 37B correspond to cross-sectionalviews taken along dashed-dotted line X1-X2 in FIG. 36B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

(Structure with EL Element as Display Element)

First, a structure that uses an EL element as a display element will bedescribed below with reference to FIG. 37A. In the followingdescription, an example of using an EL element that emits white lightwill be described; however, the EL element is not limited to thiselement. For example, EL elements that emit light of different colorsmay be included so that the light of different colors can be emittedfrom adjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 10⁻⁵g/(m²·day), preferably lower than or equal to 10⁻⁶ g/(m²·day) can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the EL element, aflexible substrate 2510 b, and an adhesive layer 2510 c for attachingthe insulating layer 2510 a and the flexible substrate 2510 b to eachother. The substrate 2570 is a stacked body including an insulatinglayer 2570 a for preventing impurity diffusion into the EL element, aflexible substrate 2570 b, and an adhesive layer 2570 c for attachingthe insulating layer 2570 a and the flexible substrate 2570 b to eachother.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, an acrylic resin, polyurethane, an epoxyresin, or a resin having a siloxane bond can be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 37A, the sealing layer2560 can also serve as an optical element.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, an EL element 2550 can be provided ina region surrounded by the substrate 2510, the substrate 2570, thesealing layer 2560, and the sealant. Note that an inert gas (such asnitrogen or argon) may be used instead of the sealing layer 2560. Adrying agent may be provided in the inert gas so as to adsorb moistureor the like. For example, an epoxy-based resin or a glass frit ispreferably used as the sealant. As a material used for the sealant, amaterial which does not transmit moisture or oxygen is preferably used.

The display device 2501 illustrated in FIG. 37A includes a pixel 2505.The pixel 2505 includes a light-emitting module 2580, the EL element2550, and a transistor 2502 t that can supply electric power to the ELelement 2550. Note that the transistor 2502 t functions as part of thepixel circuit.

The light-emitting module 2580 includes the EL element 2550 and acoloring layer 2567. The EL element 2550 includes a lower electrode, anupper electrode, and an EL layer between the lower electrode and theupper electrode.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with the ELelement 2550 and the coloring layer 2567.

The coloring layer 2567 is positioned in a region overlapping with theEL element 2550. Accordingly, part of light emitted from the EL element2550 passes through the coloring layer 2567 and is emitted to theoutside of the light-emitting module 2580 as indicated by an arrow inFIG. 37A.

The display device 2501 includes a light-blocking layer 2568 on thelight extraction side. The light-blocking layer 2568 is provided so asto surround the coloring layer 2567.

The coloring layer 2567 is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength range, a colorfilter for transmitting light in a green wavelength range, a colorfilter for transmitting light in a blue wavelength range, a color filterfor transmitting light in a yellow wavelength range, or the like can beused. Each color filter can be formed with any of various materials by aprinting method, an inkjet method, an etching method using aphotolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer 2521 covers the transistor 2502 t and the like. Notethat the insulating layer 2521 has a function of covering the roughnesscaused by the pixel circuit to provide a flat surface. The insulatinglayer 2521 may have a function of suppressing impurity diffusion. Thiscan prevent the reliability of the transistor 2502 t or the like frombeing lowered by impurity diffusion.

The EL element 2550 is formed over the insulating layer 2521. Apartition 2528 is provided so as to overlap with an end portion of thelower electrode of the EL element 2550. Note that a spacer forcontrolling the distance between the substrate 2510 and the substrate2570 may be formed over the partition 2528.

A scan line driver circuit 2504 includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a printed wiring board (PWB).

Any of the transistors described in the above embodiments may be used asone or both of the transistors 2502 t and 2503 t. The transistors usedin this embodiment each include an oxide semiconductor film which ishighly purified and in which formation of oxygen vacancies issuppressed. In the transistors, the current in an off state (off-statecurrent) can be made small. Accordingly, an electrical signal such as animage signal can be held for a longer period, and a writing interval canbe set longer in an on state. Accordingly, the frequency of refreshoperation can be reduced, which leads to an effect of suppressing powerconsumption. In addition, the transistors used in this embodiment canhave relatively high field-effect mobility and thus are capable of highspeed operation. For example, with such transistors which can operate athigh speed used for the display device 2501, a switching transistor of apixel circuit and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, by using a transistor which can operate athigh speed in a pixel circuit, a high-quality image can be provided.

(Structure with Liquid Crystal Element as Display Element)

Next, a structure including a liquid crystal element as a displayelement is described below with reference to FIG. 37B. In thedescription below, a reflective liquid crystal display device thatperforms display by reflecting external light is described; however, oneembodiment of the present invention is not limited to this type ofliquid crystal display device. For example, a light source (e.g., a backlight or a side light) may be provided to form a transmissive liquidcrystal display device or a tranflective liquid crystal display device.

The display device 2501 illustrated in FIG. 37B has the same structureas the display device 2501 illustrated in FIG. 37A except the followingpoints.

The pixel 2505 in the display device 2501 illustrated in FIG. 37Bincludes a liquid crystal element 2551 and the transistor 2502 t thatcan supply electric power to the liquid crystal element 2551.

The liquid crystal element 2551 includes a lower electrode (alsoreferred to as a pixel electrode), an upper electrode, and a liquidcrystal layer 2529 between the lower electrode and the upper electrode.By the application of a voltage between the lower electrode and theupper electrode, the alignment state of the liquid crystal layer 2529 inthe liquid crystal element 2551 can be changed. Furthermore, in theliquid crystal layer 2529, a spacer 2530 a and a spacer 2530 b areprovided. Although not illustrated in FIG. 37B, an alignment film may beprovided on each of the upper electrode and the lower electrode on theside in contact with the liquid crystal layer 2529.

As the liquid crystal layer 2529, thermotropic liquid crystal,low-molecular liquid crystal, high-molecular liquid crystal, polymerdispersed liquid crystal, ferroelectric liquid crystal, oranti-ferroelectric liquid crystal can be used. Such a liquid crystalmaterial exhibits a cholesteric phase, a smectic phase, a cubic phase, achiral nematic phase, an isotropic phase, or the like depending onconditions. In the case of employing a horizontal electric field modeliquid crystal display device, liquid crystal exhibiting a blue phasefor which an alignment film is unnecessary may be used. In the casewhere a liquid crystal exhibiting a blue phase is used, an alignmentfilm is not necessarily provided, so that rubbing treatment is alsounnecessary. Accordingly, electrostatic discharge damage caused by therubbing treatment can be prevented and defects and damage of the liquidcrystal display device in the manufacturing process can be reduced.

The spacers 2530 a and 2530 b are formed by selectively etching aninsulating film. The spacers 2530 a and 2530 b are provided in order tocontrol the distance between the substrate 2510 and the substrate 2570(the cell gap). Note that the spacers 2530 a and 2530 b may havedifferent sizes from each other and are preferably have a columnar orspherical shape. Although the spacers 2530 a and 2530 b are provided onthe substrate 2570 side in the non-limiting structure in FIG. 37B, theymay be provided on the substrate 2510 side.

The upper electrode of the liquid crystal element 2551 is provided onthe substrate 2570 side. An insulating layer 2531 is provided betweenthe upper electrode and the coloring layer 2567 and the light-blockinglayer 2568. The insulating layer 2531 has a function of covering theroughness caused by the coloring layer 2567 and the light-blocking layer2568 to provide a flat surface. As the insulating layer 2531, an organicresin film may be used, for example. The lower electrode of the liquidcrystal element 2551 functions as a reflective electrode. The displaydevice 2501 illustrated in FIG. 37B is of a reflective type whichperforms display by reflecting external light at the lower electrode andmaking the light pass through the coloring layer 2567. Note that in thecase of forming a transmissive liquid crystal display device, atransparent electrode is provided as the lower electrode.

The display device 2501 illustrated in FIG. 37B includes an insulatinglayer 2522. The insulating layer 2522 covers the transistor 2502 t andthe like. The insulating layer 2522 has a function of covering theroughness caused by the pixel circuit to provide a flat surface and afunction of forming roughness on the lower electrode of the liquidcrystal element. In this way, roughness can be formed on the surface ofthe lower electrode. Therefore, when external light is incident on thelower electrode, the light is reflected diffusely at the surface of thelower electrode, whereby visibility can be improved. Note that in thecase of forming a transmissive liquid crystal display device, astructure without such roughness may be employed.

<5-3. Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 38 . FIG. 38 corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 36B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film containing graphene may be used aswell. The film containing graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various pattern forming techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond, andan inorganic insulating material such as silicon oxide, siliconoxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. A light-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<5-4. Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 39A. FIG. 39A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 36A.

In the touch panel 2000 illustrated in FIG. 39A, the display device 2501described with reference to FIG. 37A and the touch sensor 2595 describedwith reference to FIG. 38 are attached to each other.

The touch panel 2000 illustrated in FIG. 39A includes an adhesive layer2597 and an anti-reflective layer 2569 in addition to the componentsdescribed with reference to FIG. 37A.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2569 is positioned in a region overlappingwith pixels. As the anti-reflective layer 2569, a circularly polarizingplate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 39A will be described with reference to FIG. 39B.

FIG. 39B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 39B differs from the touch panel 2000illustrated in FIG. 39A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567 is positioned under the EL element 2550. The ELelement 2550 illustrated in FIG. 39B emits light to the side where thetransistor 2502 t is provided. Accordingly, part of light emitted fromthe EL element 2550 passes through the coloring layer 2567 and isemitted to the outside of the light-emitting module 2580 as indicated byan arrow in FIG. 39B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 39A or FIG. 39B, light may be emitted from thelight-emitting element through either or both of the substrates 2510 and2570.

<5-5. Driving Method of Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 40A and 40B.

FIG. 40A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 40A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 40A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 40A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 40B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 40A. In FIG. 40B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 40B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). Sensed current values of the wirings Y1 to Y6 are shown asthe waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change in accordance with changes inthe voltages of the wirings X1 to X6. The current value is decreased atthe point of approach or contact of a sensing target and accordingly thewaveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<5-6. Sensor Circuit>

Although FIG. 40A illustrates a passive type touch sensor in which onlythe capacitor 2603 is provided at the intersection of wirings as a touchsensor, an active type touch sensor including a transistor and acapacitor may be used. FIG. 41 illustrates an example of a sensorcircuit included in an active type touch sensor.

The sensor circuit in FIG. 41 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is applied to a gate of the transistor 2613. A voltage VRESis applied to one of a source and a drain of the transistor 2613, andone electrode of the capacitor 2603 and a gate of the transistor 2611are electrically connected to the other of the source and the drain ofthe transistor 2613. One of a source and a drain of the transistor 2611is electrically connected to one of a source and a drain of thetransistor 2612, and a voltage VSS is applied to the other of the sourceand the drain of the transistor 2611. A signal G1 is applied to a gateof the transistor 2612, and a wiring ML is electrically connected to theother of the source and the drain of the transistor 2612. The voltageVSS is applied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 41 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to the node n connected to the gate of the transistor 2611.Then, a potential for turning off the transistor 2613 is applied as thesignal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied to the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, any of the transistorsdescribed in the above embodiments can be used. In particular, it ispreferable to use any of the transistors described in the aboveembodiments as the transistor 2613 because the potential of the node ncan be held for a long time and the frequency of operation ofresupplying VRES to the node n (refresh operation) can be reduced.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments and the examples.

Embodiment 6

In this embodiment, a display module, electronic devices, and a displaydevice which include a semiconductor device of one embodiment of thepresent invention will be described with reference to FIG. 42 , FIGS.43A to 43G, and FIGS. 44A and 44B.

<6-1. Display Module>

In a display module 8000 illustrated in FIG. 42 , a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011are provided between an upper cover 8001 and a lower cover 8002.

The oxide semiconductor film or semiconductor device of one embodimentof the present invention can be used for, for example, the display panel8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight 8007 includes light sources 8008. Note that although astructure in which the light sources 8008 are provided over thebacklight 8007 is illustrated in FIG. 42 , one embodiment of the presentinvention is not limited to this structure. For example, a structure inwhich the light sources 8008 are provided at an end portion of thebacklight 8007 and a light diffusion plate is further provided may beemployed. Note that the backlight 8007 need not be provided in the casewhere a self-luminous light-emitting element such as an organic ELelement is used or in the case where a reflective panel or the like isemployed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<6-2. Electronic Device>

FIGS. 43A to 43G illustrate electronic devices. These electronic devicescan each include a housing 9000, a display portion 9001, a speaker 9003,an operation key 9005 (including a power switch or an operation switch),a connection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 43A to 43G can have avariety of functions, for example, a function of displaying a variety ofinformation (a still image, a moving image, a text image, and the like)on the display portion, a touch panel function, a function of displayinga calendar, the date, the time, and the like, a function of controllingprocessing with a variety of software (programs), a wirelesscommunication function, a function of being connected to a variety ofcomputer networks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a storage medium and displaying the program or data on the displayportion, and the like. Note that functions of the electronic devicesillustrated in FIGS. 43A to 43G are not limited thereto, and theelectronic devices can have a variety of functions. Although notillustrated in FIGS. 43A to 43G, the electronic devices may each have aplurality of display portions. The electronic devices may each have acamera or the like and a function of taking a still image, a function oftaking a moving image, a function of storing the taken image in astorage medium (an external storage medium or a storage mediumincorporated in the camera), a function of displaying the taken image onthe display portion, and the like.

The electronic devices illustrated in FIGS. 43A to 43G will be describedin detail below.

FIG. 43A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible and thus can be incorporated along the curved surface of thehousing 9000. Furthermore, the display portion 9001 includes a touchsensor, and operation can be performed by touching a screen with afinger, a stylus, or the like. For example, by touching an icondisplayed on the display portion 9001, an application can be started.

FIG. 43B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, an information browsing system, andthe like. Specifically, the portable information terminal 9101 can beused as a smartphone. Note that the speaker 9003, the connectionterminal 9006, the sensor 9007, and the like, which are not illustratedin FIG. 43B, can be positioned in the portable information terminal 9101as in the portable information terminal 9100 illustrated in FIG. 43A.The portable information terminal 9101 can display characters and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons, or simply, icons) canbe displayed on one surface of the display portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed onanother surface of the display portion 9001. Examples of the information9051 include display indicating reception of an incoming e-mail, socialnetworking service (SNS) message, call, or the like; the title andsender of an e-mail, SNS message, or the like; the date; the time;remaining battery; the strength of an antenna; and the like. Instead ofthe information 9051, the operation buttons 9050 or the like may bedisplayed in the position where the information 9051 is displayed.

FIG. 43C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in the position that can be seen from abovethe portable information terminal 9102. Thus, the user can see thedisplay without taking out the portable information terminal 9102 fromthe pocket and decide whether to answer the call.

FIG. 43D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is curved, and display can be performed on the curveddisplay surface. The portable information terminal 9200 can employ nearfield communication conformable to a communication standard. Forexample, hands-free calling can be achieved with mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication. Moreover, the portable information terminal 9200includes the connection terminal 9006, and data can be directlytransmitted to and received from another information terminal via aconnector. Charging through the connection terminal 9006 is alsopossible. Note that the charging operation may be performed by wirelesspower feeding without using the connection terminal 9006.

FIGS. 43E, 43F, and 43G are perspective views of a foldable portableinformation terminal 9201. FIG. 43E is a perspective view of thefoldable portable information terminal 9201 that is opened. FIG. 43F isa perspective view of the foldable portable information terminal 9201that is being opened or being folded. FIG. 43G is a perspective view ofthe foldable portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region provides high browsability. The display portion 9001 ofthe portable information terminal 9201 is supported by three housings9000 joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from the opened state to the folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

FIGS. 44A and 44B are perspective views of a display device including aplurality of display panels. Note that the plurality of display panelsare wound in the perspective view in FIG. 44A, and are unwound in theperspective view in FIG. 44B.

A display device 9500 illustrated in FIGS. 44A and 44B includes aplurality of display panels 9501, a hinge 9511, and a bearing 9512. Theplurality of display panels 9501 each include a display region 9502 anda light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacentdisplay panels 9501 are provided so as to partly overlap with eachother. For example, the light-transmitting regions 9503 of the twoadjacent display panels 9501 can be overlapped each other. A displaydevice having a large screen can be obtained with the plurality ofdisplay panels 9501. The display device is highly versatile because thedisplay panels 9501 can be wound depending on its use.

Moreover, although the display regions 9502 of the adjacent displaypanels 9501 are separated from each other in FIGS. 44A and 44B, withoutlimitation to this structure, the display regions 9502 of the adjacentdisplay panels 9501 may overlap with each other without any space sothat a continuous display region 9502 is obtained, for example.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic device that does not have a display portion.The structure in which the display portion of the electronic devicedescribed in this embodiment is flexible and display can be performed onthe curved display surface or the structure in which the display portionof the electronic device is foldable is described as an example;however, the structure is not limited thereto, and a structure in whichthe display portion of the electronic device is not flexible and displayis performed on a plane portion may be employed.

The structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments and the examples.

Embodiment 7

In this embodiment, a deposition apparatus which can be used formanufacturing the display module of one embodiment of the presentinvention will be described with reference to FIG. 45 .

FIG. 45 illustrates a deposition apparatus 3000 which can be used formanufacturing the display module of one embodiment of the presentinvention. Note that the deposition apparatus 3000 is an example of abatch-type ALD apparatus.

<7-1. Structural Example of Deposition Apparatus>

The deposition apparatus 3000 described in this embodiment includes adeposition chamber 3180 and a control portion 3182 connected to thedeposition chamber 3180 (see FIG. 45 ).

The control portion 3182 includes a control unit (not illustrated) whichsupplies control signals and flow rate controllers 3182 a, 3182 b, and3182 c to which the control signals are supplied. For example,high-speed valves can be used as the flow rate controllers.Specifically, flow rates can be precisely controlled by using ALD valvesor the like. The control portion 3182 also includes a heating mechanism3182 h which controls the temperatures of the flow rate controllers andpipes.

The flow rate controller 3182 a is supplied with a control signal, afirst source material, and an inert gas and has a function of supplyingthe first source material or the inert gas in accordance with thecontrol signal.

The flow rate controller 3182 b is supplied with a control signal, asecond source material, and an inert gas and has a function of supplyingthe second source material or the inert gas in accordance with thecontrol signal.

The flow rate controller 3182 c is supplied with a control signal andhas a function of connecting to an evacuation unit 3185 in accordancewith the control signal.

<Source Material Supply Portion>

A source material supply portion 3181 a has a function of supplying thefirst source material and is connected to the flow rate controller 3182a.

A source material supply portion 3181 b has a function of supplying thesecond source material and is connected to the flow rate controller 3182b.

A vaporizer, a heating unit, or the like can be used as each of thesource material supply portions. Thus, a gaseous source material can begenerated from a solid or liquid source material.

Note that the number of source material supply portions is not limitedto two and may be three or more.

[Source Material]

Any of a variety of substances can be used as the first source material.For example, a volatile organometallic compound, a metal alkoxide, orthe like can be used as the first source material. Any of a variety ofsubstances which react with the first source material can be used as thesecond source material. For example, a substance which contributes to anoxidation reaction, a substance which contributes to a reductionreaction, a substance which contributes to an addition reaction, asubstance which contributes to a decomposition reaction, a substancewhich contributes to a hydrolysis reaction, or the like can be used asthe second source material.

Furthermore, a radical or the like can be used. For example, plasmaobtained by supplying a source material to a plasma source, or the likecan be used. Specifically, an oxygen radical, a nitrogen radical, or thelike can be used.

The second source material combined with the first source material ispreferably a source material which reacts at a temperature close to roomtemperature. For example, a source material which reacts at atemperature higher than or equal to room temperature and lower than orequal to 200° C., preferably higher than or equal to 50° C. and lowerthan or equal to 150° C., is preferable.

[Evacuation Unit]

The evacuation unit 3185 has an evacuating function and is connected tothe flow rate controller 3182 c. Note that a trap for capturing thesource material to be evacuated may be provided between an outlet port3184 and the flow rate controller 3182 c. The evacuated gas or the likeis removed by using a removal unit.

[Control Portion]

The control portion 3182 supplies the control signals which control theflow rate controllers, a control signal which controls the heatingmechanism, or the like. For example, in a first step, the first sourcematerial is supplied to a surface of a process member. Then, in a secondstep, the second source material which reacts with the first sourcematerial is supplied. Accordingly, a reaction product of the firstsource material and the second source material can be deposited onto asurface of a process member 3010.

Note that the amount of the reaction product to be deposited onto thesurface of the process member 3010 can be controlled by repetition ofthe first step and the second step.

Note that the amount of the first source material to be supplied to theprocess member 3010 is limited by the maximum possible amount ofadsorption on the surface of the process member 3010. For example,conditions are selected so that a monomolecular layer of the firstsource material is formed on the surface of the process member 3010, andthe formed monomolecular layer of the first source material is reactedwith the second source material, whereby a significantly uniform layercontaining the reaction product of the first source material and thesecond source material can be formed.

Accordingly, a variety of materials can be deposited on a surface of theprocess member 3010 even when the surface has a complicated structure.For example, a film having a thickness greater than or equal to 3 nm andless than or equal to 200 nm can be formed on the process member 3010.

In the case where, for example, a small hole called a pinhole or thelike is formed in the surface of the process member 3010, the pinholecan be filled by depositing a material into the pinhole.

The remainder of the first source material or the second source materialis evacuated from the deposition chamber 3180 with use of the evacuationunit 3185. For example, the evacuation may be performed while an inertgas such as argon or nitrogen is introduced.

[Deposition Chamber]

The deposition chamber 3180 includes an inlet port 3183 from which thefirst source material, the second source material, and the inert gas aresupplied and the outlet port 3184 from which the first source material,the second source material, and the inert gas are evacuated.

The deposition chamber 3180 includes a support portion 3186 which has afunction of supporting one or a plurality of process members 3010, aheating mechanism 3187 which has a function of heating the one orplurality of process members 3010, and a door 3188 which has a functionof opening or closing to load and unload the one or plurality of processmembers 3010.

For example, a resistive heater, an infrared lamp, or the like can beused as the heating mechanism 3187. The heating mechanism 3187 has afunction of heating up, for example, to 80° C. or higher, 100° C. orhigher, or 150° C. or higher. The heating mechanism 3187 heats the oneor plurality of process members 3010 to a temperature higher than orequal to room temperature and lower than or equal to 200° C., preferablyhigher than or equal to 50° C. and lower than or equal to 150° C.

The deposition chamber 3180 may also include a pressure regulator and apressure detector.

[Support Portion]

The support portion 3186 supports the one or plurality of processmembers 3010. Accordingly, an insulating film, for example, can beformed over the one or plurality of process members 3010 in eachtreatment.

<7-2. Example of Film>

An example of a film which can be formed with the deposition apparatus3000 described in this embodiment will be described.

For example, a film including an oxide, a nitride, a fluoride, asulfide, a ternary compound, a metal, or a polymer can be formed.

For example, the film can be formed with a material including aluminumoxide, hafnium oxide, aluminum silicate, hafnium silicate, lanthanumoxide, silicon oxide, strontium titanate, tantalum oxide, titaniumoxide, zinc oxide, niobium oxide, zirconium oxide, tin oxide, yttriumoxide, cerium oxide, scandium oxide, erbium oxide, vanadium oxide,indium oxide, or the like.

For example, the film can be formed with a material including aluminumnitride, hafnium nitride, silicon nitride, tantalum nitride, titaniumnitride, niobium nitride, molybdenum nitride, zirconium nitride, galliumnitride, or the like.

For example, the film can be formed with a material including copper,platinum, ruthenium, tungsten, iridium, palladium, iron, cobalt, nickel,or the like.

For example, the film can be formed with a material including zincsulfide, strontium sulfide, calcium sulfide, lead sulfide, calciumfluoride, strontium fluoride, zinc fluoride, or the like.

For example, the film can be formed with a material which includes anitride containing titanium and aluminum, an oxide containing titaniumand aluminum, an oxide containing aluminum and zinc, a sulfidecontaining manganese and zinc, a sulfide containing cerium andstrontium, an oxide containing erbium and aluminum, an oxide containingyttrium and zirconium, or the like.

This embodiment can be combined with any of the other embodiments andthe examples in this specification as appropriate.

Example 1

In this example, the crystallinity and composition of the oxidesemiconductor film of one embodiment of the present invention wereevaluated. Note that Samples A1 to A5 and Samples B1 to B5 werefabricated in this example. Samples A1 to A5 are comparative oxidesemiconductor films, and Samples B1 to B5 are the oxide semiconductorfilms of one embodiment of the present invention.

<1-1. Samples A1 to A5 and Samples B1 to B5>

First, Samples A1 to A5 and Samples B1 to B5 fabricated in this exampleare described.

Samples A1 to A5 and Samples B1 to B5 had a structure where a100-nm-thick oxide semiconductor film was formed over a glass substrate.Samples A1 to A5 and Samples B1 to B5 are different in depositionconditions of the oxide semiconductor films and compositions of thetargets. Table 4 shows main deposition conditions of Samples A1 to A5and Samples B1 to B5. Note that Samples A1 to A5 and Samples B1 to B5were fabricated with a parallel-plate-type sputtering apparatus. An ACpower source was used as a power source that supplied power to thetarget during fabrication of Samples A1 to A5 and Samples B1 to B5.

TABLE 4 Target composition Substrate Deposition Gas flow rate O₂/In:Ga:Zn temperature power Pressure Ar O₂ (Ar + O₂) × [atomic ratio] [°C.] [kW] [Pa] [sccm] [sccm] 100 Sample A1 1:1:1.2 170 2.5 0.6 180 20 10Sample A2 140 60 30 Sample A3 100 100 50 Sample A4 60 140 70 Sample A5 0200 100 Sample B1 4:2:4.1 170 2.5 0.6 180 20 10 Sample B2 140 60 30Sample B3 100 100 50 Sample B4 60 140 70 Sample B5 0 200 100

As shown in Table 4, Samples A1 to A5 for comparison were fabricatedusing a target having an atomic ratio of In:Ga:Zn=1:1:1.2 and Samples B1to B5 of one embodiment of the present invention were fabricated using atarget having an atomic ratio of In:Ga:Zn=4:2:4.1.

<1-2. Target Composition and Film Composition>

Next, the composition of the target used for Samples A1 to A5 in Table 4and the film composition of Sample A3 fabricated using the target wereanalyzed. The composition of the target used for Samples B1 to B5 andthe film composition of Sample B3 fabricated using the target were alsoanalyzed. Table 5 shows the composition of the target used for SamplesA1 to A5 and the film composition of Sample A3. Table 6 shows thecomposition of the target used for Samples B1 to B5 and the filmcomposition of Sample B3.

TABLE 5 Target composition Film composition [atomic ratio] [atomicratio] In Ga Zn O In Ga Zn O 1 1.01 1.24 4.26 1 0.84 0.92 4.04

TABLE 6 Target composition Film composition [atomic ratio] [atomicratio] In Ga Zn O In Ga Zn O 4 1.99 4.07 13.06 4 1.8 3.2 12.6

Pieces of the targets were evaluated by ICP-MS to analyze the targetcomposition. The films formed by a sputtering method were evaluated byan X-ray photoelectron spectroscopy (XPS) to analyze the filmcomposition.

As shown in Table 5 and Table 6, the film composition of Sample A3fabricated using the target having an atomic ratio of In:Ga:Zn=1:1:1.2was around In:Ga:Zn=1:1:1. The film composition of Sample B3 fabricatedusing the target having an atomic ratio of In:Ga:Zn=4:2:4.1 was aroundIn:Ga:Zn=4:2:3.

<1-3. Crystallinity Evaluation by XRD>

Next, the crystallinity of Samples A1 to A5 and Samples B1 to B5 wereevaluated by XRD. FIG. 46 shows the XRD results. For the XRD evaluation,a multifunction thin film material evaluation X-ray diffractometer, D8DISCOVER Hybrid, manufactured by Bruker AXS was used. The XRD resultsshown in FIG. 46 are results of analysis by an out-of-plane method.

As shown in FIG. 46 , a peak was observed at around 2θ=31° in each ofSamples A1 to A5 and B1 to B5. This peak at around 2 θ=31° is derivedfrom the (009) plane of the InGaZnO₄ crystal, which indicates thatcrystals in the oxide semiconductor film in each Sample had c-axisalignment, and that the c-axes were aligned in a direction substantiallyperpendicular to the formation surface or the top surface of the oxidesemiconductor film. The intensity of the peak around 2 θ=31° in SamplesB1 to B5 is higher than that in Samples A1 to A5. Furthermore, in bothSamples for comparison (Samples A1 to A5) and Samples of one embodimentof the present invention (Samples B1 to B5), the intensity of the peakat around 2θ=31° is higher under the condition where the oxygen flowrate is higher.

As shown in FIG. 46 , Samples A3 to A5 for comparison have a peak ataround 2θ=36°. The peak at around 2θ=36° is derived from a spinel phase.In contrast, Samples B1 to B5 of one embodiment of the present inventiondo not have a peak corresponding to a spinel phase in the film even whenthe oxygen flow rate during deposition is increased. This indicates thata spinel phase is less likely to be formed in the oxide semiconductorfilm formed using a target having an atomic ratio of In:Ga:Zn=4:2:4.1than the oxide semiconductor film formed using a target having an atomicratio of In:Ga:Zn=1:1:1.2.

<1-4. Crystallinity Evaluation Using Cross-Sectional TEM Image>

Next, cross-sectional TEM images of Samples A3 and B3 fabricated in theabove manner were observed. FIGS. 47A and 47B were cross-sectional TEMimages. FIG. 47A is a cross-sectional TEM image of Sample A3, and FIG.47B is a cross-sectional TEM image of Sample B3.

In FIGS. 47A and 47B, a clear lattice image of crystals having c-axisalignment is observed in Sample B3 of one embodiment of the presentinvention compared with Sample A3. It was confirmed that the oxidesemiconductor film formed using the target having an atomic ratio ofIn:Ga:Zn=4:2:4.1 had high crystallinity.

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments andthe other examples.

Example 2

In this example, the crystallinity of the oxide semiconductor film ofone embodiment of the present invention was evaluated. Samples C1 to C4,Samples D1 to D4, Samples E1 and E2, and Samples F1 to F6 werefabricated in this example. Samples C1 to C4 and Samples F1 to F6 arethe oxide semiconductor films of one embodiment of the presentinvention, and Samples D1 to D4 and Samples E1 and E2 are oxidesemiconductor films for comparison.

<2-1. Samples C1 to C4, Samples D1 to D4, Samples E1 and E2, and SamplesF1 to F6>

First, samples fabricated in this example are described.

Samples C1 to C4 had a structure where a 100-nm-thick oxidesemiconductor film was formed over a glass substrate. Samples D1 to D4had a structure where a 100-nm-thick oxide semiconductor film was formedover a glass substrate. Samples E1 and E2 had a structure where a100-nm-thick oxide semiconductor film was formed over a glass substrate.Samples F1 to F6 had a structure where a 50-nm-thick first oxidesemiconductor film was formed over a glass substrate and a 50-nm-thicksecond oxide semiconductor film was formed over the first oxidesemiconductor film. The above samples are different in the substratetemperature during deposition, the composition of the target for formingthe oxide semiconductor film, and the stacked-layer structure. Table 7and Table 8 show main deposition conditions of Samples C1 to C4, SamplesD1 to D4, Samples E1 and E2, and Samples F1 to F6. Each sample wasformed with a parallel-plate-type sputtering apparatus. An AC powersource was used as a power source that supplied power to the targetduring fabrication of each sample. A glass substrate with a size of 600mm×720 mm was used for each sample.

TABLE 7 Target composition Substrate Deposition Gas flow rate O₂/In:Ga:Zn temperature power Pressure Ar O₂ (Ar + O₂) × [atomic ratio] [°C.] [kW] [Pa] [sccm] [sccm] 100 Sample C1 4:2:4.1 100 2.5 0.6 140 60 30Sample C2 130 Sample C3 150 Sample C4 170 Sample D1 1:1:1.2 100 0.5 0.2100 100 50 Sample D2 130 Sample D3 150 Sample D4 170 Sample E1 1:3:6 130 4.5 0.3 0 100 100 Sample E2 170

TABLE 8 Stacked-layer Substrate temperature structure [° C.] Sample F1(4:2:3\1:1:1) = 100 Sample F2 50 nm\50 nm 130 Sample F3 150 Sample F4170 Sample F5 (4:2:3\1:3:6) = 130 Sample F6 50 nm\50 nm 170 Note thatvalues in parentheses represent atomic ratios of In:Ga:Zn.

In Samples F1 to F4 in Table 8, the deposition conditions of the firstoxide semiconductor film (In:Ga:Zn=4:2:3 [atomic ratio]) are the same asthose of Samples C1 to C4 in Table 7, and the deposition conditions ofthe second oxide semiconductor film (In:Ga:Zn=1:1:1 [atomic ratio]) arethe same as those of Samples D1 to D4 in Table 7. In Samples F5 and F6in Table 8, the deposition conditions of the first oxide semiconductorfilm (In:Ga:Zn=4:2:3 [atomic ratio]) are the same as those of Samples C1to C4 in Table 7, and the deposition conditions of the second oxidesemiconductor film (In:Ga:Zn=1:1:1 [atomic ratio]) are the same as thoseof Samples E1 and E2 in Table 7.

<2-2. Crystallinity Evaluation by XRD>

Next, the crystallinity of each sample was evaluated by XRD. Thecoordinate points in a glass substrate subjected to XRD are shown inFIG. 48 . FIG. 48 shows the coordinate points in the glass substratewith a size of 600 mm×720 mm subjected to XRD. The coordinate pointscorrespond to B, E, and H represented by circles in FIG. 48 .

FIG. 49 , FIG. 50 , FIG. 51 , FIG. 52 , and FIG. 53 show XRD results.XRD was carried out by a measurement apparatus similar to that inExample 1. The XRD results shown in FIG. 49 , FIG. 50 , FIG. 51 , FIG.52 , and FIG. 53 are results of analysis by an out-of-plane method. FIG.49 shows XRD results of Samples C1 to C4, FIG. 50 shows XRD results ofSamples D1 to D4, FIG. 51 shows XRD results of Samples E1 and E2, FIG.52 shows XRD results of Samples F1 to F4, and FIG. 53 shows XRD resultsof Samples F5 and F6.

As shown in FIG. 49 , FIG. 50 , FIG. 51 , FIG. 52 , and FIG. 53 , eachof Samples C1 to C4 and F1 to F6 of one embodiment of the presentinvention had a peak at around 2θ=31°, which indicates CAAC-OS, but didnot have a peak at around 2θ=36°, which is derived from a spinel phase.

As shown in Table 7, Table 8, FIG. 49 , FIG. 50 , FIG. 51 , FIG. 52 ,and FIG. 53 , the oxide semiconductor film of one embodiment of thepresent invention has high crystallinity and c-axis alignment when thesubstrate temperature during deposition is in the range of higher thanor equal to 100° C. and lower than or equal to 170° C. In particular, inSample C4, Sample F3, and Sample F4, favorable crystallinity with littlein-plane variation was obtained within a surface with 600 mm×720 mm (B,E, and H in FIG. 48 , FIG. 49 , FIG. 50 , FIG. 51 , FIG. 52 , and FIG.53 ).

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments andthe other examples.

Example 3

In this example, the hydrogen concentrations of the targets and theoxide semiconductor films formed using the targets were analyzed.Samples G1 to G4 and Samples H1 to H4 were fabricated in this example.Samples G1 to G4 were the oxide semiconductor films of one embodiment ofthe present invention, and Samples H1 to H4 were oxide semiconductorfilms for comparison.

<3-1. Hydrogen Concentration of Target>

First, Table 9 shows the hydrogen concentrations of the targets used inthis example. Note that the hydrogen concentrations of the targets wereobtained by SIMS.

TABLE 9 Target composition Hydrogen concentration quantified In:Ga:Zn[atomic ratio] results [atoms/cm³] 4:2:4.1 1.0 × 10¹⁸ 1:1:1.2 2.0 × 10¹⁹<3-2. Samples G1 to G4 and Samples H1 to H4>

Next, samples fabricated in this example are described.

Samples G1 to G4 had a structure where a 100-nm-thick oxidesemiconductor film was formed over a glass substrate. Samples H1 to H4had a structure where a 100-nm-thick oxide semiconductor film was formedover a glass substrate. The above samples are different in the substratetemperature during deposition and the composition of the target forforming the oxide semiconductor film. Table 10 shows main depositionconditions of Samples G1 to G4 and Samples H1 to H4. Each sample wasformed with a parallel-plate-type sputtering apparatus. An AC powersource was used as a power source that supplied power to the targetduring fabrication of each sample.

TABLE 10 Target composition Substrate Deposition Gas flow rate O₂/In:Ga:Zn temperature power Pressure Ar O₂ (Ar + O₂) × [atomic ratio] [°C.] [kW] [Pa] [sccm] [sccm] 100 Sample G1 4:2:4.1 100 2.5 0.6 140 60 30Sample G2 130 Sample G3 150 Sample G4 170 Sample H1 1:1:1.2 100 0.5 0.2100 100 50 Sample H2 130 Sample H3 150 Sample H4 170<3-3. Hydrogen Concentration Evaluation in Film by SIMS>

Next, SIMS was carried out to evaluate the hydrogen concentration in afilm in each sample. FIG. 54 shows SIMS results of each sample.

As shown in FIG. 54 , the hydrogen concentration of Samples G1 to G4 ofone embodiment of the present invention was around 3.0×10¹⁹ atoms/cm³ inthe vicinity of a depth of 20 nm in the film. The hydrogen concentrationof Samples H1 to H4 for comparison was around 5.0×10¹⁹ atoms/cm³ in thevicinity of a depth of 20 nm in the film. Samples G1 to G4 of oneembodiment of the present invention were confirmed to have lowerhydrogen concentration than Samples H1 to H4.

No significant difference in the hydrogen concentration in the oxidesemiconductor film was caused by the substrate temperature duringdeposition in Samples G1 to G4 and Samples H1 to H4.

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments andthe other examples.

Example 4

In this example, transistors corresponding to the transistor 270B inFIGS. 18C and 18D were fabricated and their Id-Vg characteristics wereevaluated. In this example, Samples J1 and J2 were fabricated and usedfor evaluation. Samples J1 and J2 each included ten transistors with achannel length L of 6 μm and a channel width W of 50 μm.

The samples fabricated in this example will be described below. Notethat the reference numerals used for the transistor 270B in FIGS. 18Cand 18D are used in the following description.

<4-1. Method for Fabricating Sample J1>

First, the conductive film 204 was formed over the substrate 202. Aglass substrate was used as the substrate 202. As the conductive film204, a 100-nm-thick tungsten film was formed with a sputteringapparatus.

Next, the insulating films 206 and 207 were formed over the substrate202 and the conductive film 204. As the insulating film 206, a400-nm-thick silicon nitride film was formed with a PECVD apparatus. Asthe insulating film 207, a 50-nm-thick silicon oxynitride film wasformed with a PECVD apparatus.

The insulating film 206 was deposited as follows. First, a 50-nm-thicksilicon nitride film was deposited under the conditions where thesubstrate temperature was 350° C., a silane gas at a flow rate of 200sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas ata flow rate of 100 sccm were introduced into a chamber, the pressure wasset to 100 Pa, and an RF power of 2000 W was supplied betweenparallel-plate electrodes placed in a PECVD apparatus. Then, the flowrate of an ammonia gas was changed to 2000 sccm to deposit a300-nm-thick silicon nitride film. Finally, the flow rate of an ammoniagas was changed to 100 sccm to deposit a 50-nm-thick silicon nitridefilm.

The insulating film 207 was deposited under the conditions where thesubstrate temperature was 350° C., a silane gas at a flow rate of 20sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm wereintroduced into a chamber, the pressure was 40 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in a PECVDapparatus.

Next, the oxide semiconductor film 208 was formed over the insulatingfilm 207. As the oxide semiconductor film 208, the first oxidesemiconductor film 208 b and the second oxide semiconductor film 208 cwere successively formed in a vacuum with a sputtering apparatus. Notethat in the following description, the first oxide semiconductor film208 b is referred to as IGZO-1, and the second oxide semiconductor film208 c is referred to as IGZO-2.

A 10-nm-thick In—Ga—Zn oxide film (hereinafter referred to as an IGZOfilm) was formed as IGZO-1. IGZO-1 was deposited under the conditionswhere the substrate temperature was 170° C., an argon gas at a flow rateof 140 sccm and an oxygen gas at a flow rate of 60 sccm were introducedinto a chamber, the pressure was 0.6 Pa, and an AC power of 2500 W wasapplied to a polycrystalline metal oxide sputtering target (having anatomic ratio of In:Ga:Zn=4:2:4.1).

A 15-nm-thick IGZO film was formed as IGZO-2. IGZO-2 was deposited underthe conditions where the substrate temperature was 170° C., an argon gasat a flow rate of 100 sccm and an oxygen gas at a flow rate of 100 sccmwere introduced into a chamber, the pressure was 0.2 Pa, and an AC powerof 2500 W was applied to a polycrystalline metal oxide sputtering target(having an atomic ratio of In:Ga:Zn=1:1:1.2).

Next, the conductive films 212 a and 212 b were formed over theinsulating film 207 and the oxide semiconductor film 208. As theconductive films 212 a and 212 b, a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film weresuccessively formed in a vacuum with a sputtering apparatus.

Next, the insulating films 214 and 216 were formed over the insulatingfilm 207, the oxide semiconductor film 208, and the conductive films 212a and 212 b. As the insulating film 214, a 50-nm-thick siliconoxynitride film was formed with a PECVD apparatus. As the insulatingfilm 216, a 400-nm-thick silicon oxynitride film was formed with a PECVDapparatus. Note that the insulating films 214 and 216 were formedsuccessively in a vacuum with a PECVD apparatus.

The insulating film 214 was deposited under the conditions where thesubstrate temperature was 220° C., a silane gas at a flow rate of 50sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm wereintroduced into a chamber, the pressure was 20 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in thePECVD apparatus. The insulating film 216 was deposited under theconditions where the substrate temperature was 220° C., a silane gas ata flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of4000 sccm were introduced into a chamber, the pressure was 200 Pa, andan RF power of 1500 W was supplied between parallel-plate electrodesprovided in the PECVD apparatus.

Then, first heat treatment was performed. The first heat treatment wasperformed at 350° C. in a nitrogen gas atmosphere for 1 hour.

Next, a 5-nm-thick ITSO film was formed over the insulating film 216with a sputtering apparatus. The ITSO film was deposited under theconditions where the substrate temperature was room temperature, anargon gas at a flow rate of 72 sccm and an oxygen gas at a flow rate of5 sccm were introduced into a chamber, the pressure was 0.15 Pa, and aDC power of 1000 W was supplied to a metal oxide target(In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in a sputtering apparatus.

Next, oxygen addition treatment was performed on the oxide semiconductorfilm 208 and the insulating films 214 and 216 through the ITSO film. Theoxygen addition treatment was performed with an ashing apparatus underthe conditions where the substrate temperature was 40° C., an oxygen gasat a flow rate of 250 sccm was introduced into a chamber, the pressurewas 15 Pa, and an RF power of 4500 W was supplied for 120 secondsbetween parallel-plate electrodes provided in the ashing apparatus sothat a bias would be applied to the substrate side.

Then, the ITSO film was removed to expose the insulating film 216. TheITSO film was removed using a wet-etching apparatus in such a mannerthat etching was performed using a 5% oxalic acid solution for 300seconds and then etching was performed using a 0.5% hydrofluoric acidfor 15 seconds.

Next, the insulating film 218 was formed over the insulating film 216.As the insulating film 218, a 100-nm-thick silicon nitride film wasformed with a PECVD apparatus. The insulating film 218 was depositedunder the conditions where the substrate temperature was 350° C., asilane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introducedinto a chamber, the pressure was 100 Pa, and a 27.12 MHz high-frequencypower of 1000 W was supplied between parallel-plate electrodes providedin the PECVD apparatus.

Next, the opening 252 c reaching the conductive film 212 b and theopenings 252 a and 252 b reaching the conductive film 204 were formed.The openings 252 a, 252 b, and 252 c were formed with a dry etchingapparatus.

Next, a conductive film was formed over the insulating film 218 to coverthe openings 252 a, 252 b, and 252 c and processed to form theconductive films 220 a and 220 b. As the conductive films 220 a and 220b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. Thecomposition of a target used for forming the ITSO film was the same asthat used for forming the ITSO film described above.

Then, second heat treatment was performed. The second heat treatment wasperformed at 250° C. in a nitrogen gas atmosphere for 1 hour.

Through the above process, Sample J1 of this example was fabricated.Note that the highest temperature through the process for fabricatingSample J1 was 350° C.

<4-2. Method for Fabricating Sample J2>

Sample J2 was different from Sample J1 in deposition conditions of theoxide semiconductor film 208. The other steps were the same as the stepsfor Sample J1.

As the oxide semiconductor film 208 of Sample J2, the first oxidesemiconductor film 208 b and the second oxide semiconductor film 208 cwere successively formed in a vacuum with a sputtering apparatus. Notethat in the following description, the first oxide semiconductor film208 b is referred to as IGZO-1a, and the second oxide semiconductor film208 c is referred to as IGZO-2a.

A 10-nm-thick IGZO film was formed as IGZO-1a. IGZO-1a was depositedunder the conditions where the substrate temperature was 130° C., anargon gas at a flow rate of 140 sccm and an oxygen gas at a flow rate of60 sccm were introduced into a chamber, the pressure was 0.6 Pa, and anAC power of 2500 W was applied to a polycrystalline metal oxidesputtering target (having an atomic ratio of In:Ga:Zn=4:2:4.1).

A 15-nm-thick IGZO film was formed as IGZO-2a. IGZO-2a was depositedunder the conditions where the substrate temperature was 130° C., anargon gas at a flow rate of 100 sccm and an oxygen gas at a flow rate of100 sccm were introduced into a chamber, the pressure was 0.2 Pa, and anAC power of 2500 W was applied to a polycrystalline metal oxidesputtering target (having an atomic ratio of In:Ga:Zn=1:1:1.2).

In other words, Sample J1 and Sample J2 are different in the substratetemperature in deposition of the oxide semiconductor film 208.

Through the above process, Sample J2 of this example was fabricated.Note that the highest temperature through the process for fabricatingSample J2 was 350° C.

<4-3. Results of Id-Vg Characteristics>

Next, Id-Vg characteristics of Samples J1 and J2 were measured. FIGS.55A and 55B show the Id-Vg characteristics of Samples J1 and J2. InFIGS. 55A and 55B, the first vertical axis indicates Id (A), the secondvertical axis indicates μFE (cm²/Vs), and the horizontal axis indicatesVg (V). FIG. 55A shows Id-Vg characteristics of Sample J1 and FIG. 55Bshows Id-Vg characteristics of Sample J2.

A voltage (hereinafter also referred to as gate voltage (Vg)) applied tothe conductive film 204 that functions as the first gate electrode ofthe transistor 270B was changed from −15 V to +20 V in increments of0.25 V. A voltage (Vbg) applied to the conductive film 220 b thatfunctions as the second gate electrode of the transistor 270B waschanged from −15 V to +20 V in increments of 0.25 V. A voltage appliedto the conductive film 212 a that functions as a source electrode (thevoltage is also referred to as source voltage (Vs)) was 0 V (comm), anda voltage applied to the conductive film 212 b that functions as a drainelectrode (the voltage is also referred to as drain voltage (Vd)) was0.1 V or 20 V.

From the results in FIGS. 55A and 55B, it was confirmed that favorableelectrical characteristics were obtained at the substrate temperature indepositing the oxide semiconductor film 208 of both 170° C. and 130° C.

<4-4. Gbt Tests>

Next, reliability tests were performed on Sample J1 and Sample J2. Toevaluate the reliability, a bias-stress test (hereinafter referred to asa GBT test) was performed.

The GBT tests in this example were performed under the conditions wherethe gate voltage (Vg) was ±30 V, the drain voltage (Vd) and the sourcevoltage (Vs) were 0 V (comm), the stress temperature was 60° C., thetime for stress application was 1 hour, and two kinds of measurementenvironments, a dark environment and a photo environment (irradiationwith light having approximately 10000 lx with a white LED), wereemployed. In other words, the source electrode and the drain electrodeof the transistor were set at the same potential, and a potentialdifferent from that of the source and drain electrodes was applied tothe gate electrode for a certain time (1 hour, here).

A case where the potential applied to the gate electrode is higher thanthat of the source and drain electrodes is called positive stress, and acase where the potential applied to the gate electrode is lower thanthat of the source and drain electrodes is called negative stress. Thus,the reliability evaluation was performed under four conditions in total,i.e., positive GBT stress (Dark), negative GBT stress (Dark), positiveGBT stress (Light irradiation), and negative GBT stress (Lightirradiation). Note that positive GBT stress (Dark) can be referred to aspositive bias temperature stress (PBTS), negative GBT stress (Dark) asnegative bias temperature stress (NBTS), positive GBT stress (Lightirradiation) as positive bias illuminations temperature stress (PBITS),and negative GBT stress (Light irradiation) as negative biasilluminations temperature stress (NBITS).

FIG. 56 shows the GBT test results of Sample J1 and Sample J2. Thevertical axis in FIG. 56 indicates the amount of change in thresholdvoltage (ΔVth) of the transistor and the amount of change in shift value(ΔShift). The horizontal axis in FIG. 56 indicates the name of samples,process conditions, and the like. Note that the shift value means, inthe drain current (Id)-gate voltage (Vg) characteristics of thetransistor, the gate voltage (Vg) at a point of intersection of an axisof 1×10⁻¹² A and a tangent line of the logarithm of a drain current (Id)having the highest gradient. ΔShift means the amount of change in Shiftvalue.

From the results in FIG. 56 , the amount of change in threshold voltage(ΔVth) and the amount of change in shift value (ΔShift) were within ±2 Vin GBT tests for Samples J1 and J2. Thus, it was confirmed that atransistor including the oxide semiconductor film of one embodiment ofthe present invention had high reliability.

The structure described in this example can be combined with any of thestructures described in the embodiments and the other examples asappropriate.

Example 5

In this example, a transistor corresponding to the transistor 270B inFIGS. 18C and 18D was fabricated, and Id-Vg characteristics of thetransistor and cross sections of the transistor were evaluated.

Samples K1 to K3 described below were used for evaluation in thisexample. The transistor of Sample K1 had a channel length L of 3 μm anda channel width W of 5 μm, the transistor of Sample K2 had a channellength L of 3 μm and a channel width W of 50 μm, and the transistor ofSample K3 had a channel length L of 6 μm and a channel width W of 50 μm.Note that Samples K1 to K3 with different transistor sizes werefabricated through the same steps.

The samples fabricated in this example will be described below. Notethat the reference numerals used for the transistor 270B in FIGS. 18Cand 18D are used in the following description.

<5-1. Method for fabricating Samples K1 to K3>

First, the conductive film 204 was formed over the substrate 202. Aglass substrate was used as the substrate 202. As the conductive film204, a 100-nm-thick tungsten film was formed with a sputteringapparatus.

Next, the insulating films 206 and 207 were formed over the substrate202 and the conductive film 204. As the insulating film 206, a400-nm-thick silicon nitride film was formed with a PECVD apparatus. Asthe insulating film 207, a 50-nm-thick silicon oxynitride film wasformed with a PECVD apparatus.

The insulating film 206 was deposited as follows. First, a 50-nm-thicksilicon nitride film (hereinafter referred to as an insulating film206_1) was deposited under the conditions where the substratetemperature was 350° C., a silane gas at a flow rate of 200 sccm, anitrogen gas at a flow rate of 2000 sccm, and an ammonia gas at a flowrate of 100 sccm were introduced into a chamber, the pressure was set to100 Pa, and an RF power of 2000 W was supplied between parallel-plateelectrodes placed in a PECVD apparatus. Then, the flow rate of anammonia gas was changed to 2000 sccm to deposit a 300-nm-thick siliconnitride film (hereinafter referred to as an insulating film 206_2).Finally, the flow rate of an ammonia gas was changed to 100 sccm todeposit a 50-nm-thick silicon nitride film (hereinafter referred to asan insulating film 206_3).

The insulating film 207 was deposited under the conditions where thesubstrate temperature was 350° C., a silane gas at a flow rate of 20sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm wereintroduced into a chamber, the pressure was 40 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in a PECVDapparatus.

Next, the oxide semiconductor film 208 was formed over the insulatingfilm 207. As the oxide semiconductor film 208, the first oxidesemiconductor film 208 b and the second oxide semiconductor film 208 cwere successively formed in a vacuum with a sputtering apparatus. Notethat in the following description, the first oxide semiconductor film208 b is referred to as IGZO-1, and the second oxide semiconductor film208 c is referred to as IGZO-2.

A 10-nm-thick IGZO film was formed as IGZO-1. IGZO-1 was deposited underthe conditions where the substrate temperature was 170° C., an argon gasat a flow rate of 35 sccm and an oxygen gas at a flow rate of 15 sccmwere introduced into a chamber, the pressure was 0.2 Pa, and an AC powerof 1500 W was applied to a polycrystalline metal oxide sputtering target(having an atomic ratio of In:Ga:Zn=4:2:4.1).

A 20-nm-thick IGZO film was formed as IGZO-2. IGZO-2 was deposited underthe conditions where the substrate temperature was 170° C., an argon gasat a flow rate of 25 sccm and an oxygen gas at a flow rate of 25 sccmwere introduced into a chamber, the pressure was 0.2 Pa, and an AC powerof 500 W was applied to a polycrystalline metal oxide sputtering target(having an atomic ratio of In:Ga:Zn=1:1:1.2).

Next, the conductive films 212 a and 212 b were formed over theinsulating film 207 and the oxide semiconductor film 208.

The conductive films 212 a and 212 b have stacked-layer structure of a50-nm-thick tungsten film (hereinafter referred to as the conductivefilm 212 a_1 and the conductive film 212 b_1), a 30-nm-thick tantalumnitride film (hereinafter referred to as the conductive film 212 a_2 andthe conductive film 212 b_2), a 200-nm-thick copper film (hereinafterreferred to as the conductive film 212 a_3 and the conductive film 212b_3), a 30-nm-thick tantalum nitride film (hereinafter referred to asthe conductive film 212 a_4 and the conductive film 212 b_4), and a50-nm-thick titanium film (hereinafter referred to as the conductivefilm 212 a_5 and the conductive film 212 b_5).

The conductive films 212 a and 212 b were formed as follows. First, a50-nm-thick tungsten film, a 30-nm-thick tantalum nitride film, and a200-nm-thick copper film were formed with a sputtering apparatus. Afterthat, a resist was applied over the copper film, a desired region of thecopper film was removed with a wet etching apparatus, and the resist wasremoved. Then, a 30-nm-thick tantalum nitride film and a 50-nm-thicktitanium film were formed with a sputtering apparatus over the tantalumnitride film and the copper film. Then, a resist was applied over thetitanium film, and a desired region of the titanium film, a desiredregion of the tantalum nitride film, and a desired region of thetungsten film were removed with a dry etching apparatus.

Next, a surface of the oxide semiconductor film 208 (on the back-channelside) was cleaned. As the cleaning method, a phosphoric acid solutionobtained by diluting phosphoric acid (concentration of 85 vol %) 100times with water was applied to the oxide semiconductor film 208 and theconductive films 212 a and 212 b for 15 seconds with a spin cleaningapparatus.

Next, the insulating films 214 and 216 were formed over the insulatingfilm 207, the oxide semiconductor film 208, and the conductive films 212a and 212 b. As the insulating film 214, a 30-nm-thick siliconoxynitride film was formed with a PECVD apparatus. As the insulatingfilm 216, a 400-nm-thick silicon oxynitride film was formed with a PECVDapparatus. Note that the insulating films 214 and 216 were formedsuccessively in a vacuum with a PECVD apparatus.

The insulating film 214 was deposited under the conditions where thesubstrate temperature was 220° C., a silane gas at a flow rate of 50sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm wereintroduced into a chamber, the pressure was 20 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in thePECVD apparatus. The insulating film 216 was deposited under theconditions where the substrate temperature was 220° C., a silane gas ata flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of4000 sccm were introduced into a chamber, the pressure was 200 Pa, andan RF power of 1500 W was supplied between parallel-plate electrodesprovided in the PECVD apparatus.

Then, first heat treatment was performed. The first heat treatment wasperformed at 350° C. in a nitrogen gas atmosphere for 1 hour.

Next, a 6-nm-thick ITSO film was formed over the insulating film 216with a sputtering apparatus. The ITSO film was deposited under theconditions where the substrate temperature was room temperature, anargon gas at a flow rate of 72 sccm and an oxygen gas at a flow rate of5 sccm were introduced into a chamber, the pressure was 0.15 Pa, and aDC power of 1000 W was supplied to a metal oxide target(In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in a sputtering apparatus.

Next, oxygen addition treatment was performed on the oxide semiconductorfilm 208 and the insulating films 214 and 216 through the ITSO film. Theoxygen addition treatment was performed with an ashing apparatus underthe conditions where the substrate temperature was 100° C., an oxygengas at a flow rate of 300 sccm was introduced into a chamber, thepressure was 25.06 Pa, and an RF power of 4750 W was supplied for 60seconds between parallel-plate electrodes provided in the ashingapparatus so that a bias would be applied to the substrate side.

Then, the ITSO film was removed to expose the insulating film 216. TheITSO film was removed using a wet-etching apparatus in such a mannerthat etching was performed using a % oxalic acid solution for 300seconds and then etching was performed using a 0.5% hydrofluoric acidfor 15 seconds.

Next, the insulating film 218 was formed over the insulating film 216.As the insulating film 218, a 100-nm-thick silicon nitride film wasformed with a PECVD apparatus. The insulating film 218 was depositedunder the conditions where the substrate temperature was 350° C., asilane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introducedinto a chamber, the pressure was 100 Pa, and a 27.12 MHz high-frequencypower of 1000 W was supplied between parallel-plate electrodes providedin the PECVD apparatus.

Next, the opening 252 c reaching the conductive film 212 b and theopenings 252 a and 252 b reaching the conductive film 204 were formed.The openings 252 a, 252 b, and 252 c were formed with a dry etchingapparatus.

Next, a conductive film was formed over the insulating film 218 to coverthe openings 252 a, 252 b, and 252 c and processed to form theconductive films 220 a and 220 b. As the conductive films 220 a and 220b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. Thecomposition of a target used for forming the ITSO film was the same asthat used for forming the ITSO film described above.

Then, second heat treatment was performed. The second heat treatment wasperformed at 250° C. in a nitrogen gas atmosphere for 1 hour.

Through the above process, Samples K1 to K3 of this example werefabricated. Note that the highest temperature through the process forfabricating Samples K1 to K3 was 350° C.

<5-2. Results of Id-Vg Characteristics>

Next, Id-Vg characteristics of Samples K1 to K3 were measured. FIGS. 57Ato 57C show the Id-Vg characteristics of Samples K1 to K3. FIG. 57Ashows Id-Vg characteristics of Sample K1, FIG. 57B shows Id-Vgcharacteristics of Sample K2, and FIG. 57C shows Id-Vg characteristicsof Sample K3. In FIGS. 57A to 57C, the first vertical axis indicates Id(A), the second vertical axis indicates μFE (cm²/Vs), and the horizontalaxis indicates Vg (V).

A voltage (hereinafter also referred to as gate voltage (Vg)) applied tothe conductive film 204 that functions as the first gate electrode ofthe transistor 270B was changed from −15 V to +20 V in increments of0.25 V. A voltage (Vbg) applied to the conductive film 220 b thatfunctions as the second gate electrode of the transistor 270B waschanged from −15 V to +20 V in increments of 0.25 V. A voltage appliedto the conductive film 212 a that functions as a source electrode (thevoltage is also referred to as source voltage (Vs)) was 0 V (comm), anda voltage applied to the conductive film 212 b that functions as a drainelectrode (the voltage is also referred to as drain voltage (Vd)) was0.1 V or 20 V.

From the results in FIGS. 57A to 57C, it was confirmed that Samples K1to K3 fabricated in this example had favorable electricalcharacteristics. In particular, Samples K1 to K3 fabricated in thisexample are normally-off transistors having high mobility ofapproximately 30 cm²/Vs.

<5-3. Cross-Sectional Observation of Sample K2>

Next, the cross section of Sample K2 was observed. Note that atransmission electron microscope (TEM) was used for the cross-sectionalobservation.

FIGS. 58A and 58B are cross-sectional TEM images of Sample K2. FIG. 58Ais a cross-sectional TEM image of the entire transistor 270B that wasobserved at a magnification of 20,000 times, and FIG. 58B is across-sectional TEM image of the vicinity of the conductive film 212 athat was observed at a magnification of 50,000 times. As shown in FIGS.58A and 58B, Sample K2 fabricated in this example was confirmed to havea favorable cross-sectional shape. In particular, a copper film formedas the conductive film 212 a_3 is surrounded by the conductive film 212a_1, the conductive film 212 a_2, the conductive film 212 a_4, and theconductive film 212 a_5.

The structure described in this example can be combined with any of thestructures described in the embodiments and the other examples asappropriate.

Example 6

In this example, a transistor corresponding to the transistor 270B inFIGS. 18C and 18D was fabricated, and Id-Vg characteristics, gate BTstress test results, and Id-Vd characteristics of the transistor wereevaluated.

Sample L1 described below was used for evaluation in this example. InSample L1, transistors with a channel length L of 3 μm and a channelwidth W of 5 μm and transistors with a channel length L of 6 μm and achannel width W of 5 μm were provided.

The samples fabricated in this example will be described below. Notethat the reference numerals used for the transistor 270B in FIGS. 18Cand 18D are used in the following description.

<6-1. Method for Fabricating Sample L1>

First, the conductive film 204 was formed over the substrate 202. Aglass substrate was used as the substrate 202. As the conductive film204, a 100-nm-thick tungsten film was formed with a sputteringapparatus.

Next, the insulating films 206 and 207 were formed over the substrate202 and the conductive film 204. As the insulating film 206, a400-nm-thick silicon nitride film was formed with a PECVD apparatus. Asthe insulating film 207, a 15-nm-thick silicon oxynitride film wasformed with a PECVD apparatus.

The insulating film 206 was deposited as follows. First, a 50-nm-thicksilicon nitride film was deposited under the conditions where thesubstrate temperature was 350° C., a silane gas at a flow rate of 200sccm, a nitrogen gas at a flow rate of 2000 sccm, and an ammonia gas ata flow rate of 100 sccm were introduced into a chamber, the pressure wasset to 100 Pa, and an RF power of 2000 W was supplied betweenparallel-plate electrodes placed in a PECVD apparatus. Then, the flowrate of an ammonia gas was changed to 2000 sccm to deposit a300-nm-thick silicon nitride film. Finally, the flow rate of an ammoniagas was changed to 100 sccm to deposit a 50-nm-thick silicon nitridefilm.

The insulating film 207 was deposited under the conditions where thesubstrate temperature was 350° C., a silane gas at a flow rate of 20sccm and a dinitrogen monoxide gas at a flow rate of 3000 sccm wereintroduced into a chamber, the pressure was 40 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in a PECVDapparatus.

Next, the oxide semiconductor film 208 was formed over the insulatingfilm 207. As the oxide semiconductor film 208, the first oxidesemiconductor film 208 b and the second oxide semiconductor film 208 cwere successively formed in a vacuum with a sputtering apparatus. Notethat in the following description, the first oxide semiconductor film208 b is referred to as IGZO-1, and the second oxide semiconductor film208 c is referred to as IGZO-2.

A 10-nm-thick IGZO film was formed as IGZO-1. IGZO-1 was deposited underthe conditions where the substrate temperature was 170° C., an argon gasat a flow rate of 35 sccm and an oxygen gas at a flow rate of 15 sccmwere introduced into a chamber, the pressure was 0.2 Pa, and an AC powerof 1500 W was applied to a polycrystalline metal oxide sputtering target(having an atomic ratio of In:Ga:Zn=4:2:4.1).

A 20-nm-thick IGZO film was formed as IGZO-2. IGZO-2 was deposited underthe conditions where the substrate temperature was 170° C., an argon gasat a flow rate of 25 sccm and an oxygen gas at a flow rate of 25 sccmwere introduced into a chamber, the pressure was 0.2 Pa, and an AC powerof 500 W was applied to a polycrystalline metal oxide sputtering target(having an atomic ratio of In:Ga:Zn=1:1:1.2).

Next, first heat treatment was performed. As the first heat treatment,heat treatment was performed at 350° C. for 1 hour in a nitrogen gasatmosphere and then heat treatment was performed at 350° C. for 1 hourin a mixed atmosphere of a nitrogen gas and an oxygen gas.

Next, the conductive films 212 a and 212 b were formed over theinsulating film 207 and the oxide semiconductor film 208.

The conductive films 212 a and 212 b had a stacked-layer structure of a30-nm-thick titanium film, a 200-nm-thick copper film, and a 50-nm-thicktitanium film.

The conductive films 212 a and 212 b were formed as follows. First, a30-nm-thick titanium film and a 200-nm-thick copper film were formedwith a sputtering apparatus. After that, a resist was applied over thecopper film, a desired region of the copper film was removed with a wetetching apparatus, and the resist was removed. Then, a 50-nm-thicktitanium film was formed with a sputtering apparatus over the titaniumfilm and the copper film. Then, a resist was applied over the titaniumfilm, and a desired region of the titanium film was removed with a dryetching apparatus.

Next, a surface of the oxide semiconductor film 208 (on the back-channelside) was cleaned. As the cleaning method, a phosphoric acid solutionobtained by diluting phosphoric acid (concentration of 85 vol %) 100times with water was applied to the oxide semiconductor film 208 and theconductive films 212 a and 212 b for 15 seconds with a spin cleaningapparatus.

Next, the insulating films 214 and 216 were formed over the insulatingfilm 207, the oxide semiconductor film 208, and the conductive films 212a and 212 b. As the insulating film 214, a 30-nm-thick siliconoxynitride film was formed with a PECVD apparatus. As the insulatingfilm 216, a 400-nm-thick silicon oxynitride film was formed with a PECVDapparatus. Note that the insulating films 214 and 216 were formedsuccessively in a vacuum with a PECVD apparatus.

The insulating film 214 was deposited under the conditions where thesubstrate temperature was 220° C., a silane gas at a flow rate of 50sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm wereintroduced into a chamber, the pressure was 20 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in thePECVD apparatus. The insulating film 216 was deposited under theconditions where the substrate temperature was 220° C., a silane gas ata flow rate of 160 sccm and a dinitrogen monoxide gas at a flow rate of4000 sccm were introduced into a chamber, the pressure was 200 Pa, andan RF power of 1500 W was supplied between parallel-plate electrodesprovided in the PECVD apparatus.

Then, second heat treatment was performed. The second heat treatment wasperformed at 350° C. in a nitrogen gas atmosphere for 1 hour.

Next, a 6-nm-thick ITSO film was formed over the insulating film 216with a sputtering apparatus. The ITSO film was deposited under theconditions where the substrate temperature was room temperature, anargon gas at a flow rate of 72 sccm and an oxygen gas at a flow rate of5 sccm were introduced into a chamber, the pressure was 0.15 Pa, and aDC power of 1000 W was supplied to a metal oxide target(In₂O₃:SnO₂:SiO₂=85:10:5 [wt. %]) provided in a sputtering apparatus.

Next, oxygen addition treatment was performed on the oxide semiconductorfilm 208 and the insulating films 214 and 216 through the ITSO film. Theoxygen addition treatment was performed with an ashing apparatus underthe conditions where the substrate temperature was 100° C., an oxygengas at a flow rate of 300 sccm was introduced into a chamber, thepressure was 25.06 Pa, and an RF power of 4750 W was supplied for 60seconds between parallel-plate electrodes provided in the ashingapparatus so that a bias would be applied to the substrate side.

Then, the ITSO film was removed to expose the insulating film 216. TheITSO film was removed using a wet-etching apparatus in such a mannerthat etching was performed using a 5% oxalic acid solution for 300seconds and then etching was performed using a 0.5% hydrofluoric acidfor 15 seconds.

Next, the insulating film 218 was formed over the insulating film 216.As the insulating film 218, a 100-nm-thick silicon nitride film wasformed with a PECVD apparatus. The insulating film 218 was depositedunder the conditions where the substrate temperature was 350° C., asilane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introducedinto a chamber, the pressure was 100 Pa, and a 27.12 MHz high-frequencypower of 1000 W was supplied between parallel-plate electrodes providedin the PECVD apparatus.

Next, the opening 252 c reaching the conductive film 212 b and theopenings 252 a and 252 b reaching the conductive film 204 were formed.The openings 252 a, 252 b, and 252 c were formed with a dry etchingapparatus.

Next, a conductive film was formed over the insulating film 218 to coverthe openings 252 a, 252 b, and 252 c and processed to form theconductive films 220 a and 220 b. As the conductive films 220 a and 220b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. Thecomposition of a target used for forming the ITSO film was the same asthat used for forming the ITSO film described above.

Then, third heat treatment was performed. The third heat treatment wasperformed at 250° C. in a nitrogen gas atmosphere for 1 hour.

Through the above process, Sample L1 of this example were fabricated.Note that the highest temperature through the process for fabricatingSample L1 was 350° C.

<6-2. Results of Id-Vg Characteristics>

Next, Id-Vg characteristics of Sample L1 were measured. FIG. 59 showsthe Id-Vg characteristics of Sample L1, specifically, a transistor witha channel length L of 3 μm and a channel width W of 5 μm in Sample L1.In FIG. 59 , the first vertical axis indicates Id (A), the secondvertical axis indicates μFE (cm²/Vs), and the horizontal axis indicatesVg (V).

The measurement conditions of Id-Vg characteristics were the same asthose in Example 5.

From the results in FIG. 59 , it was confirmed that Sample L1 fabricatedin this example had favorable electrical characteristics. In particular,Sample L1 fabricated in this example is a normally-off transistor havinghigh mobility of approximately 30 cm²/Vs.

<6-3. GBT Test Results>

Next, a GBT test was performed on Sample L1 fabricated in the abovemanner. The conditions of the GBT test were the same as those in Example4. FIG. 60 shows the GBT test results of Sample L1. Specifically, atransistor with a channel length L of 3 μm and a channel width W of 5 μmin Sample L1 was measured.

From the results in FIG. 60 , the amount of change in threshold voltage(ΔVth) due to the GBT test was within ±1 V in Sample L1. Thus, it wasconfirmed that a transistor including the oxide semiconductor film ofone embodiment of the present invention had high reliability.

<6-4. Results of Id-Vd Characteristics>

Next, Id-Vd characteristics of Sample L1 were evaluated. FIGS. 61A and61B show Id-Vd characteristics of Sample L1. Note that FIG. 61A showsmeasurement results of a transistor with a channel length L of 3 μm anda channel width W of 5 μm included in Sample L1, and FIG. 61B showsmeasurement results of a transistor with a channel length L of 6 μm anda channel width W of 5 μm included in Sample L1.

Furthermore, as the measurement conditions of the Id-Vd characteristicsof the transistor with L/W=3/5 μm in Sample L1, the source potential wasset to a ground potential (GND), the drain potential was set to 10 V,and the gate potential was set to 3.16 V. Furthermore, as themeasurement conditions of the Id-Vd characteristics of the transistorwith L/W=6/5 μm in Sample L1, the source potential was set to a groundpotential (GND), the drain voltage was set to 10 V, and the gatepotential was set to 4.62 V.

FIGS. 61A and 61B show high drain current saturation characteristics ofSample L1 fabricated in this example. A transistor with increased draincurrent saturation characteristics can be preferably used as a driverFET for an organic EL display device.

The structure described in this example can be combined with any of thestructures described in the embodiments and the other examples asappropriate.

EXPLANATION OF REFERENCE

11: Area, 12: Area, 21: perpendicular line, 22: perpendicular line, 23:perpendicular line, 200: transistor, 202: substrate, 204: conductivefilm, 206: insulating film, 206_1: insulating film, 206_2: insulatingfilm, 206_3: insulating film, 207: insulating film, 208: oxidesemiconductor film, 208 a: oxide semiconductor film, 208 b: oxidesemiconductor film, 208 c: oxide semiconductor film, 212: conductivefilm, 212 a: conductive film, 212 a_1: conductive film, 212 a_2:conductive film, 212 a_3: conductive film, 212 a_4: conductive film, 212a_5: conductive film, 212 b: conductive film, 212 b_1: conductive film,212 b_2: conductive film, 212 b_3: conductive film, 212 b_4: conductivefilm, 212 b_5: conductive film, 214: insulating film, 216: insulatingfilm, 218: insulating film, 220 a: conductive film, 220 b: conductivefilm, 230: opening, 240: oxygen, 250: transistor, 251 a: opening, 251 b:opening, 252 a: opening, 252 b: opening, 252 c: opening, 260:transistor, 270: transistor, 270A: transistor, 270B: transistor, 300:target, 300 a: target, 300 b: target, 301: deposition chamber, 303 b:magnet unit, 310: backing plate, 310 a: backing plate, 310 b: backingplate, 320: target holder, 320 a: target holder, 320 b: target holder,322: target shield, 322 a: target shield, 322 b: target shield, 323:target shield, 330: magnet unit, 330 a: magnet unit, 330 b: magnet unit,330N: magnet, 330N1: magnet, 330N2: magnet, 330S: magnet, 332: magnetholder, 340: plasma, 342: member, 360: substrate, 370: substrate holder,380 a: magnetic line of force, 380 b: magnetic line of force, 390: powersource, 391: power source, 501: pixel circuit, 502: pixel portion, 504:driver circuit portion, 504 a: gate driver, 504 b: source driver, 506:protection circuit, 507: terminal portion, 550: transistor, 552:transistor, 554: transistor, 560: capacitor, 562: capacitor, 570: liquidcrystal element, 572: light-emitting element, 2000: touch panel, 2001:touch panel, 2501: display device, 2502 t: transistor, 2503 c:capacitor, 2503 t: transistor, 2504: scan line driver circuit, 2505:pixel, 2509: FPC, 2510: substrate, 2510 a: insulating layer, 2510 b:flexible substrate, 2510 c: adhesive layer, 2511: wiring, 2519:terminal, 2521: insulating layer, 2522: insulating layer, 2528:partition, 2529: liquid crystal layer, 2530 a: spacer, 2530 b: spacer,2531: insulating layer, 2550: EL element, 2551: liquid crystal element,2560: sealing layer, 2567: coloring layer, 2568: light-blocking layer,2569: anti-reflective layer, 2570: substrate, 2570 a: insulating layer,2570 b: flexible substrate, 2570 c: adhesive layer, 2580: light-emittingmodule, 2590: substrate, 2591: electrode, 2592: electrode, 2593:insulating layer, 2594: wiring, 2595: touch sensor, 2597: adhesivelayer, 2598: wiring, 2599: connection layer, 2601: pulse voltage outputcircuit, 2602: current sensing circuit, 2603: capacitor, 2611:transistor, 2612: transistor, 2613: transistor, 2621: electrode, 2622:electrode, 2700: deposition apparatus, 2701: atmosphere-side substratesupply chamber, 2702: atmosphere-side substrate transfer chamber, 2703a: load lock chamber, 2703 b: unload lock chamber, 2704: transferchamber, 2705: substrate heating chamber, 2706 a: deposition chamber,2706 b: deposition chamber, 2706 c: deposition chamber, 2751: cryotrap,2752: stage, 2761: cassette port, 2762: alignment port, 2763: transferrobot, 2764: gate valve, 2765: heating stage, 2766: target, 2766 a:target, 2766 b: target, 2767 a: target shield, 2767 b: target shield,2768: substrate holder, 2769: substrate, 2770: vacuum pump, 2771:cryopump, 2772: turbo molecular pump, 2780: mass flow controller, 2781:refiner, 2782: gas heating system, 2784: movable member, 2790 a: magnetunit, 2790 b: magnet unit, 2791: power source, 3000: depositionapparatus, 3010: process member, 3180: deposition chamber, 3181 a:source material supply portion, 3181 b: source material supply portion,3182: control portion, 3182 a: flow rate controller, 3182 b: flow ratecontroller, 3182 c: flow rate controller, 3182 h: heating mechanism,3183: inlet port, 3184: outlet port, 3185: evacuation unit, 3186:support portion, 3187: heating mechanism, 3188: door, 5100: pellet,5120: substrate, 5161: region, 5200: pellet, 5201: ion, 5202: lateralgrowth portion, 5203: particle, 5220: substrate, 5230: target, 5240:plasma, 5260: heating mechanism, 8000: display module, 8001: uppercover, 8002: lower cover, 8003: FPC, 8004: touch panel, 8005: FPC, 8006:display panel, 8007: backlight, 8008: light source, 8009: frame, 8010:printed board, 8011: battery, 9000: housing, 9001: display portion,9003: speaker, 9005: operation key, 9006: connection terminal, 9007:sensor, 9008: microphone, 9050: operation button, 9051: information,9052: information, 9053: information, 9054: information, 9055: hinge,9100: portable information terminal, 9101: portable informationterminal, 9102: portable information terminal, 9200: portableinformation terminal, 9201: portable information terminal, 9500: displaydevice, 9501: display panel, 9502: display region, 9503: region, 9511:hinge, and 9512: bearing.

This application is based on Japanese Patent Application serial no.2015-024995 filed with Japan Patent Office on Feb. 12, 2015, JapanesePatent Application serial no. 2015-187620 filed with Japan Patent Officeon Sep. 25, 2015, and Japanese Patent Application serial no. 2015-215540filed with Japan Patent Office on Nov. 2, 2015, the entire contents ofwhich are hereby incorporated by reference.

The invention claimed is:
 1. A method of manufacturing of an oxidesemiconductor film comprising steps of: forming a first film with afirst polycrystalline metal oxide target; and forming a second film overthe first film with a second polycrystalline metal oxide target, whereineach of the first film and the second film comprises In, M, and Zn,wherein the M is Al, Ga, Y, or Sn, wherein the first film has acomposition in a neighborhood of a solid solution range of anIn_(1+x)M_(1−x)O₃(ZnO)_(y) structure, wherein the second film comprisesa region having a composition in a neighborhood of a solid solutionrange of an In_(1+v)M_(1−v)O₃(ZnO)_(w) structure and having a smallerproportion of indium atoms than the first film, wherein x satisfies0<x<0.5, wherein y is approximately 1, wherein v satisfies −0.2<v<0.2,wherein w is approximately 1, wherein the first film comprises a regionwhere the In, the M, the Zn, and O atoms in total account for 99.97atomic % or more, wherein the first polycrystalline metal oxide targethas a composition in a neighborhood of In:M:Zn=4:2:4.1, wherein thesecond polycrystalline metal oxide target has a composition in aneighborhood of In:M:Zn=1:1:1.2, and wherein in the case where aproportion of the In of the first polycrystalline metal oxide target is4, a proportion of the M is greater than or equal to 1.5 and less thanor equal to 2.5 and a proportion of the Zn is greater than or equal to3.1 and less than or equal to 5.1.
 2. The method of manufacturingaccording to claim 1, wherein x is approximately 0.33, and wherein v isapproximately
 0. 3. The method of manufacturing according to claim 1,wherein the first film has an atomic ratio in a neighborhood ofIn:M:Zn=4:2:3, wherein the second film has an atomic ratio in aneighborhood of In:M:Zn=1:1:1, and wherein in the case where aproportion of the In of the first film is 4, a proportion of the M isgreater than or equal to 1.5 and less than or equal to 2.5 and aproportion of the Zn is greater than or equal to 2 and less than orequal to
 4. 4. The method of manufacturing according to claim 1, whereinthe first film comprises a crystal part, and wherein the crystal parthas c-axis alignment.
 5. The method of manufacturing according to claim1, wherein the first film comprises a region with a hydrogenconcentration of lower than 1×10²⁰ atoms/cm³.
 6. The method ofmanufacturing according to claim 1, wherein the first film comprises aregion where Fe, Ni, and Si atoms in total account for lower than 0.03atomic %.
 7. A semiconductor device comprising the oxide semiconductorfilm manufactured in the method according to claim 1.